I've been cautioned that this isn't a cure-all, but it has proven a decent, safe start point...
When the model is ready to finish paint and the tip weight box is empty, assemble ALL pieces (engine, tank, wheels, etc.) and lay it upside down on a towel - fin and nose supported.
Put the reel with your intended lines halfway between fuse CL and inboard tip. Put an empty, identical reel, w/clips, etc., the same distance out on the outboard wing. The weights of the reels and clips cancel out. Only difference is the full weight of the lines in the inboard reel..
Add tip weight at the weight-box location until the model is just about balanced (neither tip definitely falls to the table.)
Secure tip weight in the box, or epoxy it in if there's no box. Whatever, you can adjust as needed later.
What this does is balance the model spanwise on the fuse centerline for the half of line weight supported at the leadout guides. We support the other half at the handle. (Full weight at half distance = half weight at full distance, right?)
This is not final tipweight trim! Only flight can get that! This is unlikely to be heavier than final trim. It does help, though. I had one very light profile with a fairly hefty head-out engine. This method found that some inboard tip weight was needed! Final trim confirmed it!
This is a 'do it until it is done' method, not calculating weights and distances. And it makes a safe start toward finding final tipweight.