Try using 275 for the La5. I think it'd best to keep the same convergence you've trained yourself with.
This!
Convergence is a personal thing. You develop a certain picture in your head when lining up shots....... when the target is about "here" as compared to the pipper I shoot. Your picture, my picture, FLS's picture are all a bit different.
What I find works best is to film a lot of your fights. Watch the films and note the range in the far right column in the viewer of when you shoot. Not so much when you "hit" but when you shoot. Next take your favorite plane up and set your guns to that range. Bring up the target (.targetxxx where "xxx" equals your range number) and fire a few rounds. Check the hits and adjust the range to bring them as close to the center as you can get.
Knowing "when" you shoot from watching the films cuts out the "learning" when to shoot part of targeting, unless of course your shooting at 600 or more. Most guns your just wasting ammo at that range.