How OPM convert imperial DN to metric DN?

Hello All,

I'd like to know how OPM(SS6) convert imperial DN to metric DN, such as 0.75 in equals 20 mm. Because I have many spec mdb files, which is .75/.5 instead of 0.75/0.5, OPM cannot recognize .75/.5 right now, I'd like to let OPM can convert .75/.5 to 20/15. Thanks a lot.