There's no other way to do it properly, since gross pay is the "bottom line" number in all official aspects. And the income tax may be refunded anyway if a reservist has to write a check to his employer for it. If you pay $10,000 in taxes on $50,000 in income and then have to write a check for $50,000 to reimburse your employer, I suspect the entire $50,000 would be fully deductible on next year's return anyway.
It seems as if there should be an easier way to do it.