Tony,
your problem is that you want 1 = 0,6 in calculations, something I suspect
is impossible. To make calculations, you have to choose between

a) store everything as minutes (i.e. 20m = 20, 1h45m = 105)
b) store everything as hours (i.e. 20m = 0,3333, 1h45m = 1,75)

Whichever you choose, calculations become simple. If you insist on storing
things only as 20m = 0,20, 1h45m = 1,45, I think you have to write a UDF or
something to parse each value and transform to a format matching a) or b)
above before attempting to calculate anything.

HTH,
Set