@ MacReeferson
For starters, I have no experience with an 18/1, never even saw one. I just recently had a big lesson in bump clearance myself. Definitely does affect compression ratio. I had been running my 6/1 with way too much bump. On 1000hr decoke, I adjusted to proper spec for a non-COV 6/1. Bump is adjusted by altering the thickness of the cylinder base gaskets. You install more than necessary (a guess) and after torquing down and checking bump, the appropriate thickness of gaskets are removed to bring the bump into spec. Really very easy once you know it should be done.
Made all the difference in ease of starting, especially in colder weather. I also have a Dursley 6/1 that I set up properly, and it is also a very easy starter. Different spec. though, as it has a COV.
I have no idea of what's proper for an 18/1, but will caution that too small of a clearance can cause problems with valve/piston interference.
Someone here will be in a better position to advise on the actual bump spec for your engine, but I can say for sure that too much will give you low compression and too little can cause valves and pistons to be in the same place at the same time.
So, yes, it is all about compression ratio.
Not very much help, but something!
Cheers,
Hugh