One method that seems to work is from The Doctors
Step-by-Step Guide to Optimizing your Ignition, by
Christopher A. Jacobs, Ph.D., E.E.
Note: do your google search and save some money.
In the second chapter he describes his method to use
a road Dynamometer, accurate to 2%. Following his
steps and method, it is repeatable.
I like what he says here, "There is so much misinformation
and even voodoo in the area of custom-tuning that it is no
wonder even the best tuners make mistakes, especially
last minute change
-type mistakes (emphasis his).
Like every other competitive sport, it will be your mistakes
(emphasis his)that cost you more races than any amount of superior
performance on the part of your competitor."
He's got calculations, forms, and even Basic Programming Steps
for code you can write to automate this.
Hope it helps,