Motor gearing

[Roaringbadger]

Hi people
I am trying to get my head round gear ratio's and have found the charts giving final drive etc. BUT how do i know what it the appropiate final drive for a specific motor wind? I am running a B4 with a 17 turn motor so i have fitted an 84 tooth spur with a 24 tooth pinion giving me a final drive of 9.1 is this ok for the motor? and how do i know the range i.e should i fit an 81 tooth spur giving me 8.775 of a 21 tooth pinion with the 84 spur giving me 10.4? ...just looking to clue myself up so that i can gear my car appropriately.....any help appreciated