You take one shaft, go to Menards, Home Depot or Lowes and check for Poly or braided nylon cord that fits snuggly into the shaft(pull nock out, most 1/4" work with 5/16" carbons).
Take a grain scale and weigh 1 inch of cord.
Then determine length needed to get to the weight you want. If the length is less (about 2/3" of the shaft length or so), then heat your field point and pull insert out as soon as you can pull it. Heat, try to pull, heat some more, etc. you get the picture. Pull out nock. Clean broken down glue from inserts, pull a long piece of serving thread through the shaft. make a loop and pull cord through shaft until about a little bit before it would hit the insert (just measure insert lenghts and a little bit).Pull serving thread through insert, put 2P-Epoxy on insert and a dab of it on end of insert. Push insert all the way in. Pull on serving thread to seat cord on back of insert. Take sharp knife, cut serving thread in insert as far down as you can and screw in a field point with waxed threads.
Let epoxy harden with nockside down. You now permanently fixed the cord to back of insert and if you do 1/3 or 2/3 you gained more FOC.
Cost: $1 Dollar Store 5 min Epoxy
$7 cord to do at least 18 arrows (50 ft)
Weight tubes cost money and change spine. Cord is cheap and does not change spine.