I currently have a 4000, spinning reel with suffix 832 15 pound braid completely filling up the spool with no backing at all on. I’m just purchased two 2500 spinning reels and was wondering if I put a little bit of backing on them and take the braid off the 4000 would be able to fill up the two 2500 with the correct amount of braid? as always thank you  

Shouldn’t be a problem. Might have enough on there to fill 3 or 4 smaller sized reels. Preferences vary, but you’ll only need 75-100 yds to fill each smaller reel. I usually prefer a little more braid for filling reels (compared to mono/fluoro top shots) to bury the connecting knot well, though I’ve started using a wrap of tape to do this with shorter lengths and it seems to be working well.

I would think so but it depends on the spools. Look at the line capacity labeled on each one. You only need about 75 yards of braid on top of the backing 

I guess the key might be to figure out just how much backing you need compared to how much space the amount of braid you want to use occupies.  Since you have two reels, you could measure out your desired length of braid and wind it onto an empty spool.  Then connect your backing material and fill the spool while counting how many cranks it takes.  Now you know exactly how much of that particular braid and that particular backing it takes to fill that particular reel. 

 

Tie the end of the backing onto the other (empty reel) and wind it the whole deal onto it.   With the info you got doing it backwards the first time around, it'll be easy to fill the 2nd reel precisely. 

For casting - ya, 75-80yds should be more than sufficient - backfill with cheap mono.

 

My pitching reels I only put 50yds on.