We have sort of done this recently in that we just moved to a house with much higher rent (more than double our previous rent) - what I did was basically work out separately on a spreadsheet what our total monthly income is and then stuck categories in one by one and made a column to add them up.
Where YNAB has come in useful for things like this is that I can use monthly figures for things like bills, but anything like car insurance or clothing where it's either spent as a one off or spent in uneven chunks, I was able to go in (do this on the web version) and look at the income and expenses report and choose averages for some expenses.
So I started out with categories that we HAVE to pay no matter what, then added stuff which we always end up spending even if I think I shouldn't spend, and I tended to overestimate a bit just in case, and it brings you to a point where you can see - OK - those luxuries need to go, these can stay, those can stay but need cutting down a fair bit, etc.
Then I went in and adjusted targets.
We also tried to live on the lowered income for the last couple of months of our old place and put the difference into a "moving fund" which has done very well for us.