There is a gear calculator that many of us here use, it can be easily configured for SBP setups. I'll show you how:
1: Get the /ratiocalc from this post
. Save the program to your desktop.
2: Run the gearratio.exe (or whatever.exe you renamed it)
3: Enter the values in the attached picture, that's your 6th gear. These are the base numbers you can refer to.
You can probably figure out which ones to change from there...this way, you can play around with the gears whenever you want!