Forex gain/loss

Hi everyone.....

I am working in a ca firm in pune.I have been allotted to do accounting and taxation of all IT companies.

Now my question is as follows:

For eg there is a company named XYZ ltd.It received foreign inflow from co in US.So now how do i calculate forex gain/loss?

I do the billing for the company by booking the expenses and on the expenses 15% is charged and booked as service charges or software charges.So when the foreign inflow comes i have to adjust it against the sales booked.

I am not understanding the calculation .how to do it? Can anyone explain me numerically?