Step 1.

Find your desired dose using tripsit.me's dexcalc.
Make sure you change "RoboCough (mL)" to "Pure (mg)"!

Step 2.

Result

To get the desired effect of ?mg HBr you would have to take:
?mg of freebase DXM (? RoboTablets)

So you could take:
? RoboTablets (?mg freebase ≈ ? mg HBr)
or
? RoboTablets (?mg freebase ≈ ? mg HBr)