How do YOU find the hole?

I need to drill two armboards for arms that aren't commercially popular. What method / tools have you folks used to find the proper location to drill the hole to mount the arm in the armboard? Obviously, i'm looking for the most accurate method that is the simplest and costs the least : ) Sean
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PS... I'm only kidding. I'd rather spend the money to do it right than to hodge-podge together something that will chew up records and spit them out :(