btw, if you need to calculate spool capacity for different lines, this online calculator has proved very accurate: https://www.pattayafishing.net/fishing- ... estimator/
This version works great for stacking lines of different diameter, e.g. mono backing with braid on top. https://www.pattayafishing.net/advanced ... estimator/
I backed two of my braid spools with Red Label 20-lb, which works very well, because it only takes about 20 yards backing to fit my working braid. Can make it a bit lighter using Big Game nylon mono for backing.
for big diameter change, use Alberto or improved Allbright knot to get a low-profile knot that will lay flat on your spool.
The key to getting a clean knot is remember which direction you wrapped the loops, and roll the loops in the same direction while you're tightening the knot.
Do this while you're gently tightening the tag ends, and tighten the standing ends last.
This is the shallow SP spool, which just has standing mono - doesn't need backing.
Same knot works great for braid to shock tippet, and there I'll add a drop of Zap CA+ to seal the knot.