Oh, I did the paycheck spreadsheet to determine withholdings for next year, several scenarios as well. I fully expect life to change though and I'll have to switch things up when it does.
This being my first full year in the working world, and now married, I'm having the darnedest time figuring out our correct withholding for 2015. I've played around with a few scenarios, but they all involve lots of calculator work. Would you be interested in sharing your spreadsheet? :)
It's not much of a spreadsheet. I start with expected income. Then take away 2 personal exemptions. Then take away the standard deduction (we can't itemize). Then take away the $2,500 deduction for student loan interest (well, a bit less this year, finally!). Last take away what we're putting into the 401-K and pre-tax medical.
What's left I calculate the 10% tax bracket, the 15%, and the 25%.
All of the actual numbers can be found using Google search.
Once you have the total amount of your expected tax, my recommendation would be to use Paycheck-city (a popular paycheck estimator) to adjust withholdings for just one of you until you find the setting that will have about the right amount of federal taxes withheld, then set your W-4 to that amount.
Right now ours is very easy because we're on one income. But if that changes I plan on leaving DH's paycheck alone and adjusting mine to whatever is needed so we don't owe come tax time next year.
Did all of that make sense?