In 34.11, if you want to avoid heavier bolts penetrating armor more efficiently than lighter ones, then you need to make all bolts have the same density. This, combined with the fact that bolts are ridiculously heavy prompted me to make them all have the density of wood.
If you download the mod in my sig and use it as is this will result in moderate (I think about 30%) protection from iron armor, and poor protection (as in only from helms) for lower armor.
If you want all plate armor to deflect bolts all the time, you can set SHOOT_MAXVEL to 222 (equivalent to Joben's SHOOT_FORCE=20) or 288 (equivalent to Joben's SHOOT_FORCE=26). I don't think it should make much difference, as arrows will deflect off all plate armor all the time with these numbers. Maybe bronze (which is actually worse than copper) will be penetrated sometimes. Also, all armor is useless against all edged attacks (including arrows/bolts) if the material is inferior to the weapon, with addy>steel>iron=bronze>copper>silver, so even when I say it gives "100% deflections" this is only if the bolt isn't made of a superior material. But if you use Joben's settings an adventurer in full iron plate won't have to worry about arrows at all, except for the areas not covered by plate (neck, upper arms, face).
Unfortunately, due to the funky rounding error in 34.11, if you reduce bolt size, then the mass is less then 1, and gets rounded to zero, meaning that bolts are fired at SHOOT_MAXVEL, so you can't just use Joben's numbers and reduce the size. You need to calculate what value of SHOOT_MAXVEL will give you an equivalent bolt momentum (which in 34.11 is approximately equal to SHOOT_FORCE because bolt masses are about 1). For example, if you want bolts to be 10 times less massive, then you would need to set SHOOT_MAXVEL to 200 to get the equivalent if SHOOT_FORCE=20. Wood bolts are 11 times less massive, hence SHOOT_MAXVEL =222.
Again, let me know if you want me to be more specific. I can also run a quick set of tests to experimentally measure how effective armor will be if you settle on numbers you think you'd like to use.