I am in the process of building up a used Trek MTB frame (1995 Trek 830 Mountain Track) that's in good shape to serve as a commuter/touring/trekking bike (replacing at least temporarily my Surly LHT that was stolen a couple months ago) One item I hope to replace soon is the seatpost bolt. The current bolt is your basic hardware store special --- plain Jane bolt, nut and washer with lots of exposed thread that was on the frame when I bought it. It'll do the trick to hold up the seatpost for the time being, but I want to replace it with a proper seatpost binder bolt, since there are "ears" on the back of the seat tube by the seat post opening designed to take a proper binder bolt; the left side one is even notched to accept a keyed bolt/nut. The problem is I don't know which size binder bolt to get.

Are there general rules of thumb for determining seatpost binder bolt length? All the threads I've come across in searches so far boil down to measuring the one already on the bike. Well, I don't have one, so I'm kinda stuck. I measured the ears with digital calipers and they're 27mm or 28mm from outside edge to outside edge (can't remember for sure; did that measurement the other night). I'm assuming that the seatpost bolt is measured from the bottom of the flange on one side to the bottom of the flange on the other when screwed all the way down. I'm also assuming that since the flanges sit down inside the the recessed parts of the ears, I don't need a 27mm size, or else the threads are likely to "bottom out" and not produce enough tension to hold up the seatpost. I think I need either a 19mm or a 22mm, but I'm not sure how much to subtract from the 27mm measurement to really get a good fit, which is why I am asking here.

Anyone with any ideas generally, or perhaps a mid 1990s vintage Trek MTB that can illuminate this issue based on what's on your bike?