Did you figure it out?

For 3 disks, the perfect score is to do it in 7 moves... That's 2^3-1.
For 4 disks, the perfect score is to do it in15 moves... That's 2^4-1.
For 5 disks, the perfect score is to do it in 31 moves... That's 2^5-1.
For 6 disks, the perfect score is to do it in 63 moves... That's 2^6-1.
For 7 disks, the perfect score is to do it in 127 moves... That's 2^7-1.