Howard, your calculations are correct.

IP address 10.10.10.2 and mask 255.255.254.0 would give you:
Subnet address: 10.10.10.0
Broadcast address: 10.10.11.255

All addresses in between are valid IP addresses, including 10.10.10.255 and 10.10.11.0

Jens, one question occurs to me. I can't see in the instructions where you are limiting the scope of the masks, so for a network of "10.10.10.0" you could apply a netmask of "10.10.10.255", which would be valid and give you a range of 168,430,336 potential addresses.

I assume that you are only using masks with contiguous bits.