I was looking at this earlier this morning. Using the rough guide from the bike shop I bought this from, it should be pretty close to the right length boom, but obviously something isn't quite right. I'm getting similar leg extension to what I get on my upright, which I've never had any issues on.

I'll see if I can move the boom out 1/4" or 1/2" and see what happens. I might need to lengthen the chain though.