To locate where an airplane balances, there are a lot of methods.

To figure out where the CG should be for your first flight, the easiest method is to measure and plug the measurements into the geistware link provided in the 2nd post above. The location is a function of the size and location of the horizontal tail of the airplane and the chord and area of the wing. All those matter. And it's easier to use the geistware.com online application to take all that into consideration. Can you get by with just figuring out where 30% is on the wing? Yeah, IF your model is very average in almost every way. And if it is, then balancing on the spar is even simpler. And probably will be as good a figuring just 30% of the MAC you took the time to find and figure out. And if simple is more important than safe, have at it. Because if your models layout is average enough that 30% MAC works, it's probably average enough that the spar is along that line. You're relying on probability, right? to save you what.... So save some more and just use the spar. And get about as "accurate" results.

To figure the mean aerodynamic chord itself requires you to measure your wing and do a fair amount of drawing of diagrams. Save yourself the time to find paper and pencil and drawing out your planform. Make the same measurements on the tail you're going to have to make on the wing, plus one, and plug those 9 measurements into the geistware application and click the button. You already had to find the yardstick, so use it for a half minute more.

Last time I figured out MAC on paper it took me longer to figure out what scale to use drawing the measurements and projections on paper than it does to boot up my pc. And finding pencil and paper took longer than that did.

Find a yardstick, measure 9 things, type that into your pc, and click a button. Found paper and pencil yet?