That's right about 2.5 gallons at 19mpg city. Sounds like a pretty solid estimate. That would mean that when the light comes on there's about 13.5 gallons of space for blend. That'd be 4.7 gallons of E85 and 8.8 gallons of 93. To throw another wrench into the equation, I'm pretty sure my city has type 2 E85 for the next few months which is E77 essentially so I reckon about 5-5.5 gallons of E85 currently and the rest 93 would give me about as close to a true 35-65 ratio as I'll get without getting crazy about it.
To be on the safe side I think I'll try 4 gallons of e85 at indicator light and rest 93 and see if any problems arise. If not I'll try for a more true 5.5 gallons of e85 and rest 93 come dyno day.