W-4 Underpayment Penalties

I'm trying to withhold as little as possible so that I take home as much as possible.  I have a stay-at-home wife and four dependent kids.  Based on what I understand, I'm withholding enough to pay my taxes each year but not enough each quarter, so I incur an underpayment penalty when I file in February. How do I calculate what I should be paying each quarter to avoid the penalty?