Trial an error. Your spools should have a mfg. line yard capacity for test # of mono for full spool. If it's like 150 yrds of 4# mono then I suppose you eye up on the spool to figure when to stop or measure it with the end of a ruler or something like a toothpick. Just mark it on toothpick as you place it on edge of spool. So if marking like the halfway mark, you should be able to fill the spool up with 50 yrds of 4# line no problem and still have some spacing for the spool edge to lip.
Another thing I personally don't like backing up a spool with thick line is that the fishing line needs to fill in the cracks and over the knot to smooth out. That may potentially take away from the main fishing line althought it's only ice fishing and we only need a few yards of line to fish with. But I like my line lay on the spool to be as smooth and even as possible, just my preference.
