Widgit,
Using the above info of 3.14159 in/rev and the desire to have 1500 IPM, your will need 1500/3.14159 =~ 477 RPM of the pinion. Now according to Mariss you base your design on max of 80% of your servo max rpm so using the 4000 rpm figure above again, we have a design value of .8x4000 or 3200 rpm. So your servo to pinion ratio will be 3200/477 ~ 6.7 and you can vary this somewhat base on what you have available to you! Your resolution with then be based on the encoder quadrature value that you use and depending on its placement (end of servo or on pinion).
Hope this helps.