If you really need to know exact, use the same calculation as you use on the arrow.
Just add the weight to the arrow in your setup and you have the answer.
The problem with that though is that you need to know exactly how much the loop, peep and serving material weighs in order to get a trully accurate number. One good reason to have a grain scale handy.
I noticed an 8-9 fps increase in speed when I removed the loop, extra tied-in nockset and peep sight....I am using a Shurz A Peep...medium size.