Or should I look at it all as generic "units", where the x might move 1.000 unit, and the a moves 90.000 units?
That is correct. Mach looks at the units. Your feed rate is the surface speed of your tool, nothing else. Say you have a rod that is going to have one spiral flute cut in it 1 inch from tip to tip. You want your Code to look like this if you are set up in degrees in your a and say your x is the axis it is running in. Your code would look like this. G1 X1.0 A360.0 F? This F value is surface speed. If you put in a 1 then it will take, I don't know, it is not important. The thing that is important is the surface speed. When you do the math to get the desired Feed rate for the cutter, that is the f value. If your surface speed needs to be 30 IPM then F=30. When you enter you Use diameter as feedrate, Mach does the math for you. I hope we are talking about the same thing. I have sent you a personal message.
Brett