Map scales are decided by the cartographer based on what they think is the the best scale for the map they're producing. This is naturally a judgment call sort of like the projection of the map. If you feel like the cartographer's choice is unbearable then I'd suggest you find a different map that covers the same territory. I believe you can find complete topo maps for all or virtually all of the lower 48 United States offered by the Bureau of Land Management offices. These maps are more general purpose maps so their scales and projections are going to be fairly standard. Also this is a big source of maps so the scales and projections they support will be broadly supported by tooling.
After a little bit of web searching I wasn't able to locate a 1:18,000 UTM corner rule. I did find a company that offers a broad variety of UTM corner rules including one at 1:35,000. This is a kilometer scale, but you should be able to easily convert that to miles if needed.