Hiya, Brian. Don't mean to barge in here, but thought I might add...
If your subtotals have no dependents, then you're gonna be okay. But if you have any type of time-math downstream where this is gonna cause trouble, just do a custom format on the cell, setting the number format to [mm]
<sup>edit</sup> Uy! Never mind! That'd be relevant if you were riding the other direction. If you're dividing then you're converting decimals to time values; in which case, I don't think this is an issue. Sorry. <sub>/edit</sub>