After giving this some thought, and validating the assessment on VMware Workstation the following has been determined.

It appears that the Assignment of Classless Inter-Domain Routing (CIDR) blocks is being used to determine the IP/Subnet Mask for the Virtual Network.  So in short the subnet Network ID must fall within the last assigned active bit of the IP (network address/host address/subnet mask).  Plus, VMware requires that at least last [4th] octet of the Host ID contains a zero.

So to use your example 10.8.241.0:

You could adjust the virtual network IP and subnet to get close to what your requirements are, and then use the VMs to further divide them up into the desired subnets.  You could use 10.8.0.0/16 which would give you the 65,534 available addresses, which might be a tad bit excessive within an VM environment, unless you're trying to emulate an existing service for testing purposes.

Another example based on 192.168.40.0

I hope that this all makes sense.