That's probably the best way to do it without a content analyzer. On mine, I'll run it until it's as empty as I think I'm going to get it, then I put E100 in it to balance it out, and my content analyzer still reads E75ish, I have to keep cycling to get to E85, takes about another 1/2 tank of E100. Even draining the car from the return into a bucket until the car dies leaves 5% gas in there.I've switched back and forth a couple of times. I run it as close to empty as I can and then fill the tank with the other fuel. As a precaution, I don't go WOT until I've cycled through a full tank.
A better approach would be to keep a couple of empty plastic five gallon drums in the trunk and when you get low on gas, fill the drums instead of the tank and then keep driving until you run out or get down to where the range displayed shows as "--" and then fill the tank from the drums.