Obviously its an empirical formula which expects the quantities in exactly the same units as you had defied them. Specifically you must use feet when defining the stemming length l.s, but centimetre when you define the density. Also the documentation seems to tell you, that the result hast to be interpreted as length in feet.
Formulas like these are not unit consistent which is disappointing and disadvantageous.
To deal with formulas like this in Mathcad, you will have to strip off the units completely by dividing each quantity by the exact unit the formula is expecting and then add the result unit at the end. That way you get the 4.76 feet you'd obviously like to see.
As Luc I suppose that the 0.333 should be 1/3 (or the third root), and so the result rather would be 4.75 feet
In this case you may also notice that phi.b can be cancelled and is not needed:
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