I wouldn't risk that. They probably meant "at the fuselage." It may be true that the MAC is nearer to the center of each wing half, but what reason do we have for thinking that the person writing the instructions would use that point as the place for you to measure the CG at? I've never seen a plan that asked you to do it that way. And even if that's wrong, the only risk from starting at the widest point on the wing is that your plane may be pretty sluggish on the first flight. The farther out you go on a tapered wing, the farther aft the CG will be, and if you get it too far back your first flight could be your last. Best to start with whichever choice gives you the most forward CG.

You can double check by looking up an on-line CG calculator. Just plug in some measurements for your plane and it will give you a reasonable starting point. A search on RCU will turn one up.