Hi Craig,
Most flybarless setups will utilise a ball link spacing of around 10mm/10.5mm on the servo output arms so i would aim to start there with your setup so you can get a good initial mechanical setting.
With regards to tape selection, i don't acutally use a BeastX myself but i know most that do will use either the BeastX foam that has the cut-out in the center (Beast Foam part number: BTXA76008) or a 3M alternative which is slightly thinner, i believe the 3M code for it is (4011).
Hope this helps!
Cheers!
Ross