You need to calculated the flare height and add the flare height
to the stack height. For example, if the stack is 35m height and the flare
height is 5m, you can introduce an stack height equal to 40m as input
data.

To calculate the flare height L, you can use the next equation

L=0.006 Q^(0.478)

being Q the heat power. The heat power can be calculated from the flux
(g/s) of methane (for example) or another gas. We can also help you to
determine this value, depending the gas you are using.

References:

Pag. 167, Fundamentals of stack gas dispersion, Milton R. Beychok,
California 1995.