Put the tab on your finger and grab the string like your going to shoot.
Look and see how much tab is past your fingers, then trim a little(I use scissors).
Then grab the string again with the tab and see if you trimmed enough, if not then trim a little more. Remember trim a LITTLE at a time.
You can trim until the tab is just a hair past your fingers or even with your fingers. This is when you grab the string.
I wear an XLarge glove but I buy a medium tab and I only have to trim a little. Hopes this helps.