I think a good ballpark starting point for 40 size on .015" lines is about 1oz; and for 60 size on .018" lines is about 1.5oz.
But I like to hang model from nose and tail with outboard wing tip on scale and make it heavier by .75oz for 40 size; or 1oz for 60 size.

you may want to do something a bit different, credit Pat Johnston for this super simple idea,,
with the airplane ready to fly ( minus lines) balance it on the spinner and the rudder, put a scale under the outboard tip and then add weight until it actually weighs one ounce. this will make up for any inherent imbalance due to mufflers, heavy wood , uneven finish weights. it will put an actual ounce on the tip.