Willie make s a valid point, one might consider building arrows and assembling arrows may be two different things.
so a bunch of this stuff you will know already- but i will just fire away.
all wooden shafts are measured on 26" centers, so cutting your shaft will not affect the static spine- unless you go less than 26 of course
now shortening will affect the dynamic spine of course.
one can quite accurately affix weight values to most arrow components, so you can get quite close to an estimated arrow weight, before you even start.
this is how i build my arrows, granted i am using pre-made parallel shafting.
1.have an accurate draw weight on my bow for my draw length- i check that!
2.taking the draw weight, style of bow and my draw length i refer to my chart for getting a starting point in static spine.
3.put one coat of sealer on, put a nock on and field point, and bareshaft them
4.once i have a static spine and arrow length tuned , i thengo ahead and build the arrows.
5.a good idea to group tune after building a few. (bareshaft, fletched field point and fletched broadhead) they should all be hitting the same spot.
some points to remember.
for the average to good shooter, out to 20 yards, it will be hard to tell 30 or 40 grains difference in mass between the shafts- there is still significant velocity at that stage, and the gravitational difference is still minimal at that stage.
i believe that having the same foc makes a more significant difference on point of impact than most things( spine excluded)- i did cover that in another post on here titled " interesting"
http://www.primitivearcher.com/smf/index.php/topic,62862.0.htmlremember when bareshafting, to have the arrows still showing slightly weak, as adding the fletching and cresting and crown will stiffen the dynamic spine.
now the better tuned your arrows are, the smaller fletch you can use, which changes the spine less, is affected by windage less, increases velocity, possibly less noisy, but it will be less forgiving of form errors.
its all a trade off
after building the arrows, i dont go back and weigh them, with the same components etc, thats not a big issue. but then again my average hunting distance is 5 to about 12 yards.
too me, spine is waaaaay more critical than mass. within 10 to 20 grains is just fine, i think. Unless of course i was shooting at an olympic level.
good luck, and dont forget the pics