DaveSSS,
Actually the SRAM Force moves 3mm for 3mm cable pull so that is a 1:1 ratio.
RobertKat, are you talking Shimano 7800 and Shimano 6700 RD? Then yes those should be compatible.
Again though. A J-Tek shiftmate would allow you to use the Shimano 7800 DT index and the SRAM Force RD. Click on the link I posted and follow it.