The Arc yields FP winnings from the difference between the factor of your Arc and the factor to be paid in GBs of other players. For example, if you invest in GBs of a 1.9 group, enter 1.9.
The factor to be paid only has an effect if your Arc factor is below the factor to be paid. For example, if your Arc is at level 80, it is not relevant for the calculation whether you pay into other GBs with 1.8 or 1.85.