I’m fine with 75yards of braided line on a spinning reel. That’s more than enough. It the braid/backing knot is becoming visible, put some electrical tape over it. No more snags on the cast.
When using filler spools, I sometimes go by percentage of the spool, and buy a separate spool for each type of reel. Do 50/50% on a 150 yard spool, or 33/33/33% on a 200 or 300 yarder.
I make a mark on the plastic spool when it’s full, then wind it on until it’s half empty on, let’s say, and 150-200 yard spool. Then when I replace that line, what’s remaining on the spool is equal to what I put on the first time, so it fills right to the top with no line wasted.
Once I get the backing and braid proportions dialed in, they don’t need to change. I know next year, 50% of a 150yard Sufix 832 spool of 10# line will fit perfectly on top of the backing on my spinning reel. It won’t change.