BACKING ON SPINNING REELS
You can get away with smaller reel, aka lighter setups, with this arrangement because a spinning reel will have a lot more capacity if using almost all braid vs. significant proportion of mono.
Typically I use 1/2 of a 150yd spool on each reel. If you want to spool full to help make your casts longer: add the 75yrs braid then top off with mono. Put that line on another spool and then to another spool then back on the reel you want to use. Yes, extra work but you get a full spool.