I must be a proper modeller, I've made a jig:

Actually there's not much to it but after some frustration with the performance of my Spratt & Winkle equipped stock on Paynestown, I decided to make a simple coupler height bar setting jig, and this is the not very technical result. The main problem I was having was rolling stock uncoupling when it wasn't supposed to. The idea with the jig is that there's a slot cut in the end piece into which the coupling bar should slip, if it's at the right height. The end piece has a central recess for the coupling arm to enter.

After checking all my stock, and adjusting where necessary, I've got to be honest and say that the results haven't been as dazzling as I'd hoped for. Yes, there's a general improvement in reliability, maybe from 70 to 85%, but it's not like night and day. I think the problem is that, even when you've got the height of the bar set correctly, there's still quite a lot of room for error in the other parameters, such as the distance that the bar is set out from the buffer beam, and the angle of the arm. I was hoping that by being virtuous and building a jig, I would have made the necessary observances to the model railway gods and my coupling issues would be behind me. That said, it is still a useful tool for setting up the couplers when installing them, and I'm trying to be a bit more consistent in setting up the other distances and angles, as well as revisiting some of the older stock where my installation was significantly "off".

My plan for couplings, incidentally, is more or less divided between BR and pre-nationalisation. For BR stock, it'll be mainly S&W equipped but with 3-links between short rakes or blocks of similar vehicles such as fish vans which will not be shunted. For pre-nationalisation, where the bulk of my stock is already 3-link equipped, I'll only do S&Ws on passenger vehicles.