Difficult to determine on your 250 but it seems like 90% load is best. Here is a thread by Sentra with a lot of good data that may be relevant, but I would do my own testing to determine which is the best. 90% load should work out to 10% of atmospheric vacuum reading or about 2-3 inches.
http://ecomodder.com/forum/showthrea...oad-19594.html
Of course they are completely different vehicles-engines but I think you may find the best acceleration is more a function of load that anything else, as long as you are in the right gear, neither too low or high.
regards
Mech