What is the best way to measure and wind backing and line on a new reel. The only convenient fly shop here is an orvis and I want to try some new lines.
wind the line on the reel backwards with the backing attached. Fill the reel as full as you want and cut the backing. Unspool everything and wind on the backing first. Another way is to believe the capacities written on the reel box but they always say up to a certain amount of backing and are for weight forward lines only.
Here are 2 more tips that work with the first one.
Pound a nail part way into a short piece of wood. Put the center hole of your new line’s spool over that nail and then spin the new line on your reel. When you are done with the backing attachment, use your finger to spin the line and backing back on the new line’s spool. Then tie the backing to your reel and spin the backing and line onto your reel.
Don’t try to totally fill your reel with line and backing unless you normally distribute the line across the reel evenly when landing a fish. Otherwise you risk having the line jam up on the reel before you have reeled the fish to your net/hand.
I have seen tips on how much room you should leave once you have the backing and line on your reel. You should be able to fit a pencil between the line and the reel, you should be able to easily get the pencil in without forcing it.
Kind of a PITA but that’s the only way I know to do it. That “pencil” tip is a good one; I think I’ve got some reels that are too full, and I don’t know why; I haven’t had a fish take me into the backing yet :(.
Excellent, thank you for the tips, now I just need to choose my first non-orvis line in six years 
Cortland Classic peach. Works in any temperature. Good longevity. Time tested.
I use the rule of thumb of 1/4 inch “head space” between the line and the tip of the spool. It compensates for the fact that you will never wind the line as carefully or distribute it as evenly as you do when first loading the reel. That extra head space ensures you won’t bind the reel.
I use the wind the line and backing on backwards method described by a previous poster. As a practical matter, I use the holes in the ventilated spool to judge how much space is left before the spool lip. When the backing begins to be seen at the right level of the spool you can stop and reverse the process.
This may be a dumb question, but didn’t your reel come with a chart that shows how much backing to use? All of mine did.
Some do, some I buy “vintage/used”