Frame #12 as arrowed in the photo below, but be careful not to count from the two #7 frame parts just forward of frame 12

Glen

I had this problem, as well look in the mags, can't remember which issue but you can work out the frame numbers from here. Laterly ( I should have done it earlier) I marked the very top of the frames with the numbers, as I kept forgetting, in fact at one time I was counting the frames the wrong way round. You could always try a bit of masking tape with the numbers on taped around the rib tops, as numbering becomes very important when planking the decks, the mags even tell you to mark the deck supports to make sure positioning is perfect.