There are 3 calculations you can try.   First measure the distance covered by one rotation of your wheel in metres.   You'll almost certainly have used that when you first got your cycle computer.   Then choose the distance of the time trial:

Calculation 1: Enter any values for chainwheel, rear sprocket and RPM.  The calculator will tell you what time you'd achieve.

Calculation 2: Enter any values for chainwheel, rear sprocket and target time (in minutes and seconds).   The calculator will tell you what RPM you have to do.

Calculation 3: Enter any values for chainwheel, target time and RPM.  The calculator will tell you what rear sprocket you will need to achieve that time.

Its very interesting (if, like me, you are a right "statto").  

Download here

 Incidentally, I wrote this from first principles, its my work and not copied!  You may have it, distribute it and use it freely.  However, no warranty of guarantee is offered or implied and the author and club cannot be held responsible for any misuse or abuse or injury sustained as a result of using it!