im using rbcs...the instructions said screw down tell it touch ramp then add a 1/4-1/2
Crimping is basically moving the case mouth towards the central axis of the casing.
What you are doing in pistol cartridges that headspace off the mouth (ie headspace is measured from casehead to casemouth and relies on the casemouth to be square for accurate measurement) is removing the belling or case mouth expansion you made prior/during powder charging.
Best way to know if you are doing it right is to measure your crimp.
A case mouth is generally .010" thick. Double that and add the actual bullet diameter. For 9mm and a .355" bullet, your crimp should measure .355+.01+.01=.375 with a +/- .002.
Crimping in a pistol cartridge is just flattening the case mouth along the bullet shank, nothing more.
Here's an example with .45 ACP.
Measure crimp here.
.01+.01+.451=.471
This one is a bit tight, but .469" to .471" is acceptable.