Regarding the trim length. Unless you have a custom reamer most chambers have a generous amount of clearance at end for growing cases. Why not KNOW the dimension? You can make your own or buy Sinclair's chamber plugs. Many chambers are so long that you will never have to trim your brass.

https://www.sinclairintl.com/reload...clair-chamber-length-gage-prod32925.aspx
there is a video showing how they are used.




Varmint Al wrote a good article on chamber length:
https://www.varmintal.com/arelo.htm#Chamber_Length

specs on making your own chamber length gauges if you have access to a lathe
https://www.varmintal.com/alath.htm#Chamber_Length_Gauge