Hello Monaliza,
the elevation is taken from the altitude band of the OLCI product. So, it is not necessary to specify this value.
The best exponent and factor can be found if you compare your in-situ data with the processing results in Excel. As chl_a is computed by default by this equation
21.0 ∗ (iop_apig)^1.04
you should start from the iop_apig values calculating the chl_a.
You can compare the values in a plot and try finding better factor and exponent matching your in-situ data.