I know this has been answered, but a little more information doesn't (usually) do any harm.
I'm not sure if you do this, but I find it best to create my own variables and apply values to them, do the calcs on those variables and then link them to dimensions. This way the calculation is not lost, only the link between a dimension and the variable, which can easily be re-applied.
If you do calculations based on dimension name you could lose both.
dim A = Dim B x Dim C
create variables a, b & c
assign values to b & c
assign formula b x c to a.
dim A formula = a
dim B formula = b
dim C formula = c
HP Z420 16GB RAM Quadro K4000 ST10 MP12 on Windows 10 Pro 64.