if you are just trying to "do it", then here is a very easy way...

mark your left starting edge and right starting edge, then lay a ruler
diagonal from the left edge to the right edge in such a way that the
whole numbers (1,2,3.. ) line up perfectly and divisible by whatever
spacing you want.

for example -- say you have a board that is 9.939 (the point being, it
does not matter) and you want to come in .5" from the left and .5"
from the right -- mark those left and right margins on the board. now
say you want to divide that space by 5. so you have 9.939 -1 or
8.939 of material to divide. take a ruler and place the "0" on the
left margin that you marked and just let the right side of the ruler
come down diagonally to align the 10" measurement on the ruler to the
right side margin. the ruler will now be at some angle -- does not
matter what. simple mark the 2", 4", 6", and 8" markings of the ruler
onto the material. you have just equally spaced the material between
your left and right margin into 5 equal parts. If you wanted to make
it 6 equal parts then bring the 12" ruler marking down to the right
margin line. now mark the material at 2, 4, 6, 8, and 10 -- giving
you 6 equal parts. hopefully you get the idea. you can use
whatever ruler markings you want -- 1,2,3,4 or 2,4,6,8, etc or 1.5,
3, 4.5, 6, etc so long as you use the same increment and the ruler is
on some (any) diagonal (must be diagonal -- straight across does not
work). now center up a hole on those lines and they will be
perfectly aligned. the key with circles is to work off of the center
points whenever possible.

good luck -- credit for this goes to some article i've read in the
last few months -- dont recall where -- absolutely brilliant though.