DHCP scope size to number of DHCP hosts ratio

I typically create subnets just for user workstations so they end up having about 230 IPs per /24 subnet. I keep server and network gear separate for easy ACL creation on the routers. I wouldn't recommend going much larger than this per subnet due to the number of broadcasts the workstations will generate.
The DHCP server will handle the load fine. I have one moderate server (VM) servicing about a dozen of these subnets currently with no issues.

Simply mathematics suggests that yes, 100 IP addresses is sufficient for 65 hosts.

However, what would be more telling, would be the growth trends you've experienced so far: how quickly did it get to 65 hosts? Do you expect this trend to continue? That's a 5 minute email conversation you can have with HR and it'll help immensely with your planning.

Another caveat would be wireless clients: how many wireless leases do you have/expect? Often people will bridge their wireless network onto their existing network, keeping the default lease duration of 7 days. However, if you have a steady flow of smart phones, tablets, laptops that may or may not be there the next day (i.e. guests, like clients, contractors, etc.) then you can quickly burn through your available pool if the clients are not releasing their addresses and notifying the DHCP server.

Generally speaking, I find /24s to be the most useful for everything but iSCSI/storage networks, DMZs and "glue" networks between routers (I use /29s, enough for the routers and a laptop if I need to sit on the same subnet with a laptop): its the subnet everyone knows, it's a happy size for broadcast traffic, and once you get beyond that, you're usually breaking up your network into more manageable chunks with VLANs anyways.

What is a good ratio of count of hosts to DHCP scope ip address count to avoid congestion?

I have never seen the any value in trying to use the absolute minimum. When working with RFC1918, just setup a scope with lots of available space. There really isn't any major risks or problems caused simply by having more address space available in your scope then what you need.

If you want do want to minimize the range for some reason, then the best just figure out what you need through estimation and monitoring. Make a guess, that is somewhat larger then the expected number of hosts the network will see during the lease length, then simply setup tools to monitor how many available addresses you have in the scope. Increase or decrease the sized based until you have reached your criteria.

One DHCP server can hand out hundreds of addresses a second, so with a sufficiently large scope a single server could probably serve a /8 happily. Basically don't sweat it, it'll all be just fine, unless your phone is your DHCP server anyway :)