A 1× or 1
unit key is the spacing of the letter and number keys.
You can just measure from the LHS of the keycap in question, to the LHS of the adjacent keycap, and divide that distance by the keycap-to-keycap distance of the letter keys, which is typically 0.75 in, 19.05 mm.
In the case of my Filco though, the ratio of keycap widths themselves give me 1.25 for a modifier; the keycap-to-keycap distances give me 1.26!
Either way, you can just measure it directly if you don't have a reference source such as multipla :-P