Hi Bob, sorry I can't help with your question. I don't use masking tape. I paint them free hand. I'm too scared the paint might lift off when
it would come time to removing the tape!
I draw the lines for the boot topping or any hull demarcation line with a pencil, then paint the line freehand with a small flat headed brush, or if really narrow, a very small fine pointed one.
If you fancy a go, here's a picture of how I mark around the hull.
Make sure the hull is level and true, then set the pencil at the right height. In the picture it's resting on a small plastic 'Box', then mark around the hull. Re- set your boot topping depth and around the hull you go again. Then paint between the parallel pencil lines.
Obviously this example isn't a boot topping, but the principal is the same.
Once again, sorry I can't be of more help, but I'm sure the lads will be along toot sweet with the answer you require.
Cheers,
Ron