I hold the spool beside my reel so that the line comes off the spool the same direction it goes on the reel. I do it at the top on both. Then I lay the spool on the floor and use my non winding hand to keep tension on the line while I wind the line on the reel.
I give it a couple of turns and then let the pressure off, The line should lay in loose loops and not twist. If it twist the I flip the spool over and continue.
Not too scientific, but it work for me.
Here's two links for calculating line backing which is kind of important for casting performance and will also save you money if your using braided line.
https://www.pattayafishing.net/fishing- ... estimator/https://www.igfa.org/About/Estimating-M ... ectra.aspxI've used these and they work great. I use Berkley Big Game for backing, real cheap and easy to find at CTC. I have a 100' tape and lay it out on the driveway for measuring the backing. Someday I'll get a line counter.