Clueless "Add /64" button - an option to choose the number of the subnet

Currently, "Add /64" button appears to give me a result of ($current_allocation -1). i.e. I currently have 2001:8b0:xxxx:a713::/64 - pressing Add /64 gives me 2001:8b0:xxxx:a712::/64.

Perhaps add a box to allow the user to specify their own choice of subnet address (within reason, and sense-checking)?

Same probably goes for the "Add /60".