Once the SPOUT connector is replaced, the idle timing is advanced from the 10° base. I seem to remember something in the 20° range last time I checked. Changing the base timing on the EFI trucks shifts the entire timing curve up the same amount. For example, at 10 degrees base timing, the total timing might be say 35 degrees (just as an example, I don't know an exact number that Ford used on these trucks). Then for a base timing of 15, that total timing is now shifted to 40 degrees instead of 35. That's why the base timing is used as a point of comparison.

 

Your best method is to "power time" ie the hill method makes you engine run at its most efficient with what you are currently running 87, 89, 94 etc.. It takes a little away from the bottom end probably not noticble. You can also leave the base as is and open up your advance curve to increase your "total timing" by a programmer if electronic or different springs if mechanical. Setting base timing with a timing light is just a starting safe place.