24" is the basic velocity sweet spot off the bench, kinda, for the most part, for most paper, yada...
https://rifleshooter.com/2016/02/6-5-creedmoor-effect-of-barrel-length-on-velocity-cutting-up-a-creedmoor/
For HSLD, I'd go 18" with a good suppressor. You'll be good for 1.2K if you're good...