|I looked to see if this was already posted and didn't see it, so my apologies if it is already here....
I hate marking buttons on button-front top patterns and have nearly quit doing it. Instead, here's what I do.
Complete the top except for buttons/buttonholes and try it on. Using straight pins, pin the CF together at the top and at the fullest part of your bust (this will be where the top will most likely gap if there's no button there). With another straight pin, mark those locations and remove the pins holding the shirt together. If you have a loose fitting top that only needs 4 buttons, hold the right front at the bust level pin and fold the top down so that the pin at the top meets the CF . This gives you the location for button #3; place a pin there. Now, using this level as the new fold line, fold the shirt down so that the bust-level pin meets the CF below pin #3; this is the location for the bottom button. You now have 4 evenly spaced buttonhole locations marked and you never picked up a measuring tool!
If you need more closely spaced buttons, fold the top so that the top pin meets the bustline pin and put a mark at the fold (divides distance between the two original marks). Now, fold the top at the bustline pin as before and mark where the next two pins meet the CF. You can keep folding and pinning button placement until you get as many as you need and they will all be the same distance apart.
(This is one of those things that's a lot easier to do than describe; hope it makes sense! :) )