When in doubt, start at 25% MAC and slowly move back... you usually end up on the main wing spar.

MAC is the AVERAGE chord. Easy to find for a straight un-tapered wing (every rib is MAC), an unnecessary source of confusion for many people. any HS geometry instructor can fnd an average chord. then you go 25% from front to rear on that line... you are at 25% MAC

Another way to look at it... find the line where 25% of the wing is in front and 75% behind that line... that is 25% MAC.