I have had some luck using the macros
"parserNumberWithUnits.pl",
"parserFormulaWithUnits.pl"
(See the following for a working example: NPL Directory: WHFreeman/Rogawski_Calculus_Early_Transcendentals_Second_Edition/9_Introduction_to_Differential_Equations/9.2_Models_Involving_y'=k(y-b)/9.2.8.pg)
I hope this helps...