I'm not an experienced trike rider, but that left leg looks very extended. Is that normal?
Looks to me like he might want to bring the boom in an inch or so. Assuming he's in the right place in the seat, which he probably isn't because you're never quite in the right place in the seat until you've been riding for a few minutes and have settled into it.
Also: we know nothing of his pedalling technique. Many recumbent riders ankle more than on uprights, which he may or may not do. There's also the whole pedalling one-footed thing to consider - I've no idea what that's going to do.
To me, that looks like a good starting boom position to set the chain length for. The rear mech should be able to cope with the slack as he plays about with bringing it in. What you don't want to have to do is extend the chain because you've added 5mm to the boom and it now jams if you select big:big.
