Just finished filling out this exact paperwork. There is no set amount. It's exactly the same as filling out a W-4. You set your number of dependents, etc. It also asked if you wanted state taxes deducted as well. I chose yes. You will no longer pay FICA or towards Social Security.
Also, here we don't pay towards our health insurance, both you and spouse are covered. We've been paying towards retiree medical benefits almost our whole career with an 0.89 cent/hr. wage deduction so we'd be covered in retirement. Local 315 NorCal