One important piece of information which is required is the number of revolutions it takes your speedometer to register one mile. On most modern cars, this may be found on the bottom (or elsewhere) of the speedometer face, usually in very small numbers. Look for something like 940, 960, 1000, or 1020, or something close. This number is the number of speedometer cable revolutions (or equivalent in the case of electronic sender units) to increment the odometer one mile. Most speedometers (modern ones) are 1000 revs/mile, and this is a good starting number to use.
Another vital piece of info is the speedometer drive/driven gear ratios, the gears inside of the transmission or transaxle. The drive gear is the one which is turned by engine power, usually mounted right on the transmission output shaft. The driven gear is the one which is "driven" by the "drive" gear, and is connected to the speedometer cable or electronic sender. The numbers entered in the program are the integer number of teeth - just count 'em up.
For example:
rev/mile: 1000, tire diameter 31", differential: 3.07, drive teeth: 15 (count them on your TJ) will result in a 30 tooth 'driven' gear.
Running the equation backwards keeping everything else the same and using a #27 gear you would have 35" tires.
You can also calculate using drive shaft revolutions/mile (assuming 1:1), axle ratio and tire size.
There are additional formula to use, how did you arrive at 27?
Have you contacted an 'actual' speedometer shop? There should be at least one in any fair sized city. If necessary you can contact one via phone/fax/e-mail.