Dial calipers,or micrometers,for accuracy.
Most of the marble people I know are happy if your within 1/16th,of an inch.With the exceptions being pee-wee,1/2( 0.50),or any other bench markes size.You know the ones,,over an inch,,,over 3/4,,,,over 2 inches,,,ect,,ect,,,,
If your useing a template,make sure it is accurate,to begin with,and non-flexible.If you bend it a little while you are measuring,you can have a larger marble go through,a smaller hole.