No5Mk1 150 grain chrony results

One way of getting a baseline is to lay your hands on some decent quality MkVII ball rounds, at least enough to fire some groups to show you how your rifle performs with the ammunition it was designed and built for.  I've had a couple of local Lee Enfield guys do that - contacting me to sell them 10 of the Greek HXP rounds I have remaining to fire for group.

Handloaders attempting to sort out Lee Enfields have a lot more exploration ahead of them than they do with something else like a Remchester .308 Winchester.  The scores of threads here and elsewhere in getting the action and barrel properly fitted in the furniture, the condition of the barrel, etc. are just one part of the equation.

The other part is that the Lee Enfield/Mk VII ball round is a package where each was engineered to operate with the other (generally speaking).  This is a 4 MOA package (again, generally speaking); if anything is off in the package, grouping is going to perform a lot worse.  It is far more complex than purchasing the 174gr. bullets from PPU, Sierra, Hornady, etc - for one thing, those bullets are quite different than the MkVII bullet: boattails rather than open based, and not the internal construction of the ball bullet.

If you're looking to do more than kill tin cans for recreational fun at the local borrow pit and are trying to get the best out of the rifle, there's a lot of exploring ahead.  Aside from taking a deep dive into how good the condition of the stocking is, trying reloads with different bullet designs, different weights and different powders is the easiest and quickest way to hope for quick success.