I'm not sure it really matters as long as you use the same math every time.
Let me explain: (bear with me... this is long)
If you end up with 3.42:1 as the ratio programmed in DS and 28.5" as tire height programmed in DS.... you can calculate theoretical driveshaft RPM. Let's assume for simplicity that your trap speed in DS was 100 MPH. You are using a 3.42:1 gear... and a 28.5" tire. a 28.5" tire has a circumference of Pi x dia .... 3.1416 x 28.5 = 89.54 in traveled in 1 revolution of the tire. 100 miles converted to inches is 100 (mi) x 5280 (ft/mi) x 12 (in/ft) = 6,336,000 inches in 100 miles. Now to figure out how many tire revolutions that would take..... well... you traveled 89.54" in one tire revolution..... so it would take 6,336,000 / 89.54" = 70,761.67 tire revolutions to travel 100 miles. Because you are running 3.42 gears.... the driveshaft had to turn 70,761.67 x 3.42 = 242,005 revolutions to make the car travel 100 miles. Up to this point... we have been talking about hours... mi/hr....tire rev/hr.... and driveshaft rev/hr.... now to convert that to minutes to get revolutions per minute..... 242,005 / 60 (min/hr)..... = 4033.41 driveshaft RPM.
Now..... if I hadn't lost anybody..... you can make a direct comparison to what DS said was the engine RPM at that 100 MPH..... for example if it were 4700 RPM... you would take the larger number (4700) divide by the smaller number (4033.41) = 1.165... subtract 1 ..... = .165..... then multiply by 100 to convert to percent..... so.... 16.5%.... this is the slip % you had at 100 MPH.
This all works at WOT only..... slip percentages will be all over the place at different loads... so only do the analysis at WOT.
Anyone feel free to jump in and correct my math if I made a mistake.