3/4″ is 19.05mm
18mm is only 1.05mm under whereas 22mm would be 2.95mm over
as we are not dealing with precision engineering here TBH you might get away with just converting all the dimmention straight to metric and letting the slight differences in final internal volume be as it it. Most people using hand tools will have some varience in their cut dimmentions anyway so CNC cut direct conversion to metric might not be much worse. This isnt a suggestion just a thought aloud!
Alternatively you could get any parts with critical dimmentions cut over size so you can sand down to fit perfectly upon assembly if that is something your comfortable doing