The critical thing that you have to keep in mind is that the inside dimensions stay the same and how the pieces are put together. And knowing that helps with the conversion. I use whatever calculator comes with either my computer or my phone, or the online conversions from websites such as Google, etc., to change back and forth from metric and imperial in decimal form.
3/4″ is a hair over 19 mm, so choose accordingly.
Next, figure your inside dimensions in metric, then using the assembly guide / drawing (not sure what is included) to see what pieces extend past the inner boundaries so you can add the appropriate extra for the cuts.
For the cut outs on the face plate, if you are going with the 18 mm, converted dimensions should be close enough. Just make sure that anything that is flush with the inside is measured as such.
BTW, there’s a thread somewhere in a past iteration of the forum talking about various “cut list” programs, and if you go up to the search bar and put in cut list, you’ll find a write up out on the main page of one that Nick (Toid) recommends. I believe the program can be set for either metric or imperial.
**Addendum – I looked at that program and it is in imperial measure. The good thing is that once you figure out the sizes in metric, you can then convert back to imperial and use the decimal form to set up the cut list in the program, converting back to metric one last time. With enough decimal digits, there should be little to no error in converting so many times.