The only way to do this correctly is to project your 2015 tax bill and then look up how your employer is instructed to withhold taxes from your paycheck.
+1
Do you have significant other income or itemized deductions? If not, especially given the 7 months that will have been withheld at the single rate, changing each W-4 to "Married with 1" (as suggested in the OP) should be fine and will at least reduce the size of the zero-interest loan you are giving to the IRS.
See http://forum.mrmoneymustache.com/ask-a-mustachian/how-can-we-optimize-our-withholdings/ and links therein for similar discussion.
I have a reasonable amount of deductions. Enough that I can go the itemized route, but I am not sure how it works when filing jointly... (She has basically no deductions)
I also max my 401k, but she can't, her company only allows a max of 30% contributions, which is only about 10-12k. So that pulls our incomes in different ways. (Hoping to get her a raise to 60+ so she can max it)
When itemizing as a married couple, it's the same process as before, except with two people's worth of deductions. Just claim all the deductions that you and your spouse can claim.
The difference is that the decision to take the standard deduction vs itemize is based on a new comparison: the standard deduction for a married couple is double that of a single person. So for 2015, the standard deduction is 12600.
This is one example of how getting married can hurt you in taxes - suppose you itemized and your spouse didn't before you two got married. Furthermore, your sum of itemized deductions was between 6300 and 12600. Now, after you're married, it's not advantageous for you to itemize, because you now have to overcome a standard deduction of $12600, not $6300.
If you provide more details about your tax situation we may be able to help you figure out your paycheck withholdings.