DHCP scope within a larger subnet

Hopefully someone can help me out with this, seeing how the Demo is almost over, and MGMT would like some results from this product.

Say we have an IP range... 10.66.130.0  /23

That will give 510 usable IPs.

Say we take the first hundred, and statically assign them to devices (server, switches, etc.)

Say we take the middle 200, and assign them as a DHCP scope.

Can IPAM help me determine just the number of DHCP assigned IPs that are being used?

I tried to go into my large IP range (130.0 /23) and select "IP Range" - add.

I type in the starting and ending IP range of the DHCP scope, but the save button stays greyed out.
Anyone have any suggestions?