15/64" is good place to start. 5/64" is probably a typo. If the hole is much over 1/4" the plug has less effect and the idle RPMs drop slower/hang longer. A smaller hole usually means a quicker drop, but can effect mid range pull. Performance effects vary, so drill up 2 or 3 different sizes and see which cap works best for you. They can be changed out in about 5 min.
