I don't seem to have my manual on hand, but I think I followed it when I built my tandem 6 years ago. There are 4 frames fore-to-aft and three spaces between them. Frames 1 & 2 and 3 & 4 are 36" apart and 2 & 3 are 32" apart. I drilled the monorail with 2 holes 36 inches apart, placed the rail on the first and last pair of frames and drilled through. I also set it on my other monorail and drilled holes in the latter to match. Now I can put the paired Piantedosis on in either order.

For the single configuration I bolted one monorail's 'footward' hole through the hole I'd made in frame #3. (It's hard to keep track of 'front', 'back', 'fore' and 'aft' when the rig is facing backwards!) Then I marked the monorail to match the hole in frame #2 and drilled my third hole. It was very close to 32" seatward of the footward hole. So 4 holes through the frames, 3 in one monorail and 2 in the other.

"(It's hard to keep track of 'front', 'back', 'fore' and 'aft' when the rig is facing backwards!)"

Oh, aye, unless you were born backwards like me, which might explain why I prefer rowing to paddling.  I'm like Mater (the rustbucket wrecker in the Cars animated movies): "I go backwards better'n I go forwards!"  <;-)

.....Michael

 It only gets worse - I can't tell left from right to begin with, and then one of the seats of the tandem is a forward rower, so nobody has any idea what 'aft', 'port', etc means. I put green and red tape on the grips and looms so I can say 'turn to RED'. Then I row with my brother... who's colorblind.