I used M5 as well - works fine and although you don't have a lot of adjustment range, there should be enough if you are careful. I made up a steel drill guide (a short length of steel bar with a 4.3mm hole through the middle) that had a reduced diameter one end that fitted the counterbores in the rail so it centred the drill accurately. You can then use a similar guide with a 5.1mm hole to guide the tap so that it goes in square.