Just follow the procedure to measure the “TRAVEL LENGTH” of the scale that you need.
(The physical length is 132mm(5.2inch) longer than the travel length of the scale, e.g. travel length is 800mm (31.5inch), physical length is 932mm(36.7inch).)
(We always mention the travel length for DRO as it is a more important piece of information than the physical length.)
1) Move your X axis to one extreme end and make a marking.
2) Move to the other extreme end and make another marking.
3) Measure the distance between the 2 marking, this is the TRAVEL LENGTH.
4) Select the correct travel length from our available list of travel length.
If the maximum horizontal travel of your milling machine is 420mm. Since scales come in 50mm increments, you need the next size scale which would be a 450mm scale
5) Do the same for the other axis/axes.