Without the backing you run into two issues.
1) – it just costs too darn much to fill the spool w/o the backing. I would have to guess that a guys could get hundreds of yards of unneeded braided line on a line counter to fill the spool.
2) – if you don’t fill the spool the diameter if the spool is GREATLY reduced and the amount of line retrieved with each crank of the reel handle falls off dramatically.
My suggestion is to use the backing.
Here’s a fool-proof way to determine the proper amount of backing when you have multiple reels to fill.
I always set up my trolling rods to hold one complete filler spool of braided line. In the case of fireline I believe you get 100 yards. That way when it comes time to replace the fireline I know exactly how much line needs to be replaced. The old braided line gets stripped off. You tie in where the old braid met the backing and then you re-spool.
Anyway….
Start by taping the end of the braided line to the spindle of the reel being spooled. (Yeah, I know, we’re putting it on backwards. )
Wind on the braided line using consistent tension.
Once you have all the braided on the reel, clear your linecounter (set to 000).
Tie the end of the braided line to your backing of choice. I like 10# Trilene XT.
Wind on your backing line until you get the spool to where you want it. Use consistent tension to get the line to lay properly on the spool.
IMPORTANT STEP: Make a note of the feet reading on your line counter after the mono backing has been spooled on the reel.
You now have one reel with the proper amount of line on it but the order of the braided line and mono backing is reversed.
Wind that first finished spool onto a second reel, using consistent pressure to get that line to lay nice and tight. You’ll notice the spool fills PERFECTLY and the braided line is on the working end of things where it should be.
Use the line counter reading recorded at “IMPORTANT STEP” to wind on the proper amount of backing line to the remain spools. Top off with one spool of braid per reel and you’re set.
Write down that line counter reading and the diameter of the line you used to fill your first spool and you’ll be good to go should you ever need to replace the backing. Write that info in black magic marker under the reel seat and you’ll never lose it.