Is it possible to "guesstimate" the ethanol content in your tank by looking at the long term fuel trim? Or maybe your AFRs?
TL;DR incoming wall of text - be forewarned, LOL
With AFR, no. Reason being, your ECU will use input from your primary O2 sensor to measure AFR and will adjust trims accordingly to hit whatever AFR target applies at any given moment, whether idling, @ WOT, etc. Seeing AFRs off does, however, often indicate boost/vacuum leaks because the system/logic in the ECU assumes everything is operating without mechanical issues, so your AFRs will be impacted by mechanical issues, like leaks (usually the worse the leak the bigger the impact).
LTFTs also aren't the best measure due to the fact that they're based on an average over an extended period of time. It's like the avg. speed or MPGs on your info center - the more miles you've driven, the longer they take to change because more and more data is being averaged the farther you drive. Now if you re-flash your map right when you fill up, LTFTs can be a good indicator when they start populating (takes a little while) because all of the averaged data is cleared.
STFTs are a better measure of instantaneously measuring how accurate your mix is, but even then it's a little tricky because they will also show figures as your car is adding or subtracting fuel to hit your targeted AFR. This is completely normal. So you'll see a lot of activity in STFTs, and if you're not used to the normal range they usually work within, they can be misleading.
The best method of ethanol equalization is using the same E85 pump every time and putting in a precise ethanol:regular in your tank every time you fill up. That's why I bust out the calculator to be precise. But this hinges on knowing for a fact you can fit x amount of gallons in your tank. For instance, if someone runs a 25% mix and assumes they can fit 12 gallons in, put in 3 gallons of E85, but can only fit 7 gallons of 93 (so 10 gallons total instead of the 12 they were shooting for), they just created a situation where they're running a good deal more ethanol than they intended. That's one of the reasons I always over-estimate how much fuel is remaining in my tank. This means that I have to refill more often because I rarely get the tank completely full; it also means my ethanol ratio stays on point.
Worst-case scenario, use your AP to monitor WOT AFR/KR to make sure you're still safe (not lean at WOT; no excessive KR, etc.), and if necessary, drive while staying out of a lot of boost until on fumes, go back to a full 93 tank, and start over, running it down until you can fit the proper amount of ethanol in and top up with 93.
