When weighing your letters, give them a slight spin to make the weighing platform rotate. This cuts friction between the straw and the dowel so as to give you an amazingly consistent reading. Here we see the dowel through the straw at 1/2 an ounce, so we know a single 44-cent stamp will cover it.
The USPS provides a handy tool for calculating postage
(once you've established its weight).