I've determined that my truck still has 5 gallons in the tank when the low level light comes on. So, after driving the truck home from the gas station to clear out the lines of the old fuel (7 miles), I'd determine the actual octane in the tank this way:
((5 x 87) + (25 x 93)) / 30 = 92
I think, really, you could probably switch to the 93 octane tune at this point without too much danger. Don't push the truck hard till you refill the tank with 93 though.
- Jack