You will have to use shafts that are close to the spine you need for your bow. If you can find an arrow that shoots well for you use it to compare the dowels to. You can make a simple spine tester by nailing 2 nails in the side of your work bench, long enough to support your shafts and 26" apart. Place the dowels across the nails and hang a 2# weight from the center and mark where the dowel bends to, the deflection. By matching dowels to that mark you should be pretty close. The spine is calculated for a 28" arrow with a 125 gr point.
Start with full length dowels. You can adjust the effective spine by adding or removing length by 5# per inch, ie. a dowel that spines 65# can be made to fly like the correctly spined arrow by adding 4" over 28"...so 32" arrow. You can add a heavier point to reduce the effective spine. That value is reduce or gain 5# of spine for each 25grs of tip weight over 125gr...so by adding a point that is 170gr your effective spine would be reduced by 10#.
What draw weight are you planning of making? if 45# or less go with 5/16" for 50# and up go with 3/8" unless you can get 11/32", they would be better.
You can reduce the spine by sanding the shaft. Be sure you get shafts with as few run-offs as possible, none would be the best but hard to find in dowels. Poplar makes good arrows but so does chundo, ramin and most standard dowels.
Now, that was easy wasn't it?