Before I start fitting uncoupling magnets to the railway layout in a box, I have to work out where they need to be. To do this the track has been plonked back into place, making sure the point tie bars are over the holes, and then marked out again so I put it back in the same place later.
On a micro layout you need to eek out all the siding space you can so I've been going around pushing wagons along converging tracks to see how close to the point they can be before clobbering each other. Then I mark the ends of the buffers on the board - with Spratt & Winkle uncoupling, this is the point you want the centre of the magnet. Only short wheelbase wagons will be appearing - if you plan on using bogie or particually long wheebase stock including diesels, allow half a wagon length of extra clearance so vehicles can pass without endangering fragile side detail.
On a larger model it's also worth measuring out the distance between magnets to that in a long train only one coupling is over a magnet at a time. This is fine until you introduce the odd long wheelbase vehicle when all bets are off and you can find a train splitting in two places at once. Electromagnet would solve this but I can't be bothered with the extra expense and wiring.
And yes, I know that wagon has an 'orrible big Hornby tension lock on it at the moment. This will be changed eventually, it was just the only one I had to hand...