First I would make the variables consistent. On the left hand side you use the term , but on the right hand side you just use . I would stick with just , so you have:
Note that 'R' here is a constant - namely the radius of the outlet hole.
You can rearrange to get a differential equation:
If we set the cionstant this can be written as . Rearrange:
, and integrate to get , or The value for C can be found from the initial conditions: