You can buy gadgets like this from Aliexpress for under $50.
Fill reel/spool with braid, then add required backing line to get perfect depth of line on your reel
Reverse the process winding the line back onto the plastic spool (the backing line will be on the bottom)
Use a second plastic spool to wind the line onto (the backing line will now be on top)
Place this second spool onto the gadegt
Wind onto the reel/spool
Worth it when you have multiple reels.