If the only variable were initial arrow velocity, trajectory would be fairly easy to calculate. However, the other important variable is friction loss due to air resistance. There are a number of variables that affect friction loss, such as fletching, arrow mass, and shaft flexing. Imagine the difference in trajectory between a perfectly tuned bare shaft and a flu flu, both fired at the same initial velocity!
Arrow weight doesn't directly affect trajectory, assuming the initial velocity is equal, as Newton proved when he dropped an apple and a cannon ball off the leaning tower of Pizza. (At least I think that's what he did; at least they were two things of similar size but different weights). But weight does affect momentum, which helps an arrow overcome the effects of air resistance. Imagine the difference in trajectory of shooting a soda straw and a wood arrow shaft, both with the same initial velocity. The difference would be even more if you shot a soda straw and a steel rod, but it would be hard to get a steel rod up to the same initial velocity as an arrow shaft out of a bow.
Because of the many variables in wind resistance, it is really just easier to do some kind of real life testing, as Kirk suggests. Or even just skip that and measure the gap at the target, as in Jummy Blackmon's, Rod Jenkins's, or Byron Ferguson's videos.