The approximate Mean Aerodynamic Chord (MAC) position on a swept wing is found just like it is on a tapered wing, or (tapered and swept wing).

You need to measure the wing root chort and tip chord. Ignore any wing fillets or tip shapes - they don't count. Tip chord is often best measured at the last wing rib, particularly with rounded wing tips.

You then measure aft from the leading edge at both the tip and root locations. For example if you want to find the 25% CG location and have an 8 inch root chord and a 6 inch tip chord, you would measure aft 2 inches at the root and 1.5 inches at the tip.

You then connect these points with a straight line. (A dry erase marker or piece of taunt strings works well). The point where this line intersects the midpoint of the wing panel (half way between the tip and root) is then the location of the 25% MAC of the wing. These two points can then be connected with a straight line to show the 25% chord location at the wing root.

If both wings are evenly tapered and staggered you can use the composite wing method indicated above combined with the root and tip measurments done above. Just add the stagger to the root and tip measurments assuming the same chord is used for upper and lower wings. Alternatively with unequal wing chords you'll need to estimate the measurements from the upper wing LE to the lower wing TE (with a positive staggered wing, the reverse would be used with a neagative staggered wing). You can also level the model and obtain the measurements with a square over a level surface as you mark the surface directly below the points on the wings and then measure the distance from leading and trailing edges there rather than trying to measure it accurately on the upper and lower wings on the model.

Where it gets tricky and complicated is on a Pitts type configuration with a swept upper wing and straight lower wings both of different areas. That gets hard to explain and involves more calculus from my ERAU days than I care to remember.

I would figure both the 33% MAC and the 25% MAC, mark them at the wing root and start the test flying with the CG at the 25% location and move aft toward the 33% location until you get the agility you want.