rc = Root Chord
t = Taper Ratio = (Tip Chord ÷ Root Chord)
MAC = rc x 2/3 x (( 1 + t + t2 ) ÷ ( 1 + t ))
say the root chord is 11" and the tip chord is 6"
t = 6 ÷ 11 = .5455
Now plug t into the formula to find the MAC. Note that the wingspan and sweep do not matter. No matter what the span or how much the wing is swept, the MAC will always be the same length.
MAC = 11 x 2/3 x (( 1 + .5455 + .54552 ) ÷ ( 1 + .5455 ))
MAC = 22/3 x ( 1.8431 ÷ 1.5455 )
MAC = 7.3333 x ( 1.8431 ÷ 1.5455)

MAC = 7.3333 x 1.19254

MAC = 8.7453"