Calculation of best focus point

Hi,

my question regards the calculation of the best focus point for a
situation in wich dof should extend from some near limit different
from the hyperfocal point to infinity. I know about the formula 2*X*Y/X
+Y (where X is the near limit of dof and Y is the far limit) to give
the best focus point but what value do I have to set for Y if it
should be infinity? - 0 or some negative value?

Any input very welcome!
Michael