I find I try to err on the side of a little extra for more sample room. (I have the louet table-top mill.) I find I can usually get it within +/- 12″ of my desired warp length. I don’t always place the bottom cross-beam directly across from the top one, sometimes it’s on a side adjacent, depending on how long of a warp I need. I figure as long as they’re not both on the same side, the whole thing will hold together well enough.