If you don't have a scale, you can make one real easy that is far more accurate than you would believe. Take either a wooden yard stick or similar size lath, drill a small hole at 1" and 35" line. Tie a piece of string w/ alligator clip about a foot long thru each hole. Then take a stright pin w/ a foot or so of thread on it and stick the edge at approx middle [18" on yard stick] Fool w/ this until the stick hangs level. Drill a small hole for center string now that you know where center is. Use a baggie on each side to hold the weights. Below are some common items for weights.
Field tip = 100gr [or 125]
Dollar bill = 15.34 gr
Quarter =88 gr
Nickel = 76.7 gr
Penny =38 gr
Dime = 35 gr
You have to be a little inventive, lol. But by using different combinations you can get w/ in a few grains. Just remember to put a baggie on both sides if you are weighing an arrow. Have fun!!