So after some digging, here is what I've found. Reactive ISOs do not scale directly to your level; they scale to your stats. This is why a calculation based directly on level trails upwards, because as you level you slowly get more stats added per level. At lower levels, you only get 2-4 points added per level. In the 200 range (where I am) you get more like 5-7.
The big question is: how then do you calculate ISO scaling? The best answer I have found so far is using the stats on Generalist armor. I then take the stat value on a Silver, Single Stat ISO and divide it by the trench stat and I start to see the pattern. Here are a few of the data points I have for comparison, these use the gold buy Commander's Trench:
LVL = SSS ISO / Gen Stat = Coefficient
lvl 153 = 65 / 670 = 0.09701
lvl 191 = 84 / 867 = 0.09689
lvl 195 = 86 / 888 = 0.09685
lvl 198 = 87 / 904 = 0.09624
So it looks like these are generally targeted to be 10% ISOs, based on what is considered the Agent's standard stats. I have found if you do this comparison on the Generalist's S.Tech Combat Armor at a given level, you end up right around an actual 10%. Personally, I find it easier to grab the stats from the Commander trench, but thats mostly because I own one so I don't need to fumble through the Store looking for it each time I need to update my spreadsheet. Really, you should be able to do this calculation on any Generalist armor stat and get a value you can use. If another level armor is easier for you, feel free to use it.
There is a level of fluctuation past the thousands place, but remember that the ISOs actual value has been rounded, so getting the number by reversing that calculation is not going to be precise. The coefficient I have been using on my spreadsheet is 0.0985; this seems to work well for the couple of levels I have been though since finding the connection to base stats. The numbers can be off by a point here or there, but you aren't able to control stats to that level of precision, so it should be close enough.
I have also been able to pretty well confirm my other calculations on how ISOs break down relative to each other at any given level. If a SSS ISO is our base 100%, then the stats on a Double Stat ISO will each be 80%, the stats on a Triple will each be 60% and the stats on a Chaotic will each be 40%. Gold ISOs are all 120% of their Silver counterparts. Health and Stamina as a stat are now x5 that of At/D/Ac/E on the same type of ISO (they previously came out to somewhere in the realm of x5.06, but they now seem to be a strait x5).
One other thing to keep in mind when selecting ISOs to use, Chaotics are still the jack of all stats and the master of none, but do come out as most efficient when looking at total points per ISO. 6 stats x 40% each is 240%, giving them more than double the total boost of a single stat ISO. A Double stat ends up at 2 x 80% = 160% and a Triple at 3 x 60% = 180%, meaning that overall they are about equally efficient for adding stats. So the most important thing between which to use is going to be the breakdown of stats you need to raise.
As a general rule, don't use Singles. They are horrible. I don't know of a hero that justifies slotting only one stat where using a Double is only slightly less efficient for one stat and gives you a whole second stat for free. If you really only find one stat needs to be raised, go back and look again. Personally, if I really needed that extra 20%, no way around it, I'd probably get a Gold Double. Just saying.
If you need to raise 2 stats, then Doubles seem obvious. If one of them really doesn't need to be raised that high, then I would probably look to buff another stat or 2, but still using doubles as you most likely have that one stat you are trying to max..
At 3 stats, I would start with the relative Triple, since the new silver cost Prismatics means there is now a Triple for all seasons. Depending on how you want the 3 stats distributed, you may need to look at some Doubles to max one stat above the other 2.
Into 4 stats, start with some Triples again, but keep Doubles in mind since it's completely based on which stats need how much. You may even what to look at a Chaotic or 2, especially if one of those stats doesn't need to be raised very high, as sometimes it can help you still try to maximize the other 3 stats. For example, you want to raise H/At/Ac but also a little bit of Stam. Using triples means using at least one ISO with Stam, but missing H, At or Ac. If you can get that Stam with a Chaotic or 2, you can then fill the rest with Steady (H/At/Ac) and still may be ahead in the long run.
At 5 stats, while you probably won't use strait Chaotics, I would probably start by using Chaotics till you fill the lowest stat you want to raise. Try to get the other stats you want up using Triples and Doubles, looking to replace a Chaotic or 2 if needed. For instance, if you want to add a little Stam, try a Chaotic or 2 and then use Doubles and Triples to fill out the rest. Even though you are raising a 6th stat you may not really want, those points are basically extra, anyway. Looking at it as raising only 5 stats x 40%, you still end up 20% above the total stats on a Triple. Sometimes that little extra boost means you don't have to use a whole ISO to try and cover it somewhere else.
Raising all 6 stats is a rare case because normally you are going to raise either Evasion or Defense but not both, I guess there might still be room for it on the few characters where E and D start out the same. I used to use a lot of 'Slot Chaotics and then tweak the soft spots' builds on heroes that were already pretty well balanced or were hard to get all the numbers I needed (Dr Strange, Scarlet Witch, Deadpool, etc). But I think with silver Prismatics allowing you to get all sorts of combinations that were difficult to swing before, those days are mostly behind us. Should you really think you want to raise all 6 stats, you are most likely going to need Chaotics just to get the kinds of boosts that raising all 6 stats will require. Start much like a 5 stat build, try Chaotics till your lowest stat is full and then see what you have to move around to get the other stats where you need them.