"I direct deposit half of each paycheck into her account."
That's the root of the problem. That arrangement cannot produce the intended result. By basing it on your paycheck you could cause her to receive less by simply increasing your tax withholding. If you are eligible to participate a 401(k) plan offered by your employer you could cause her to receive less by making elective deferrals to a 401(k). An equitable arrangement would not be dependent on those factors. You'll need to come up with a better arrangement to get the result that you want.
