Our aim is to stay a bit under budget and we've been doing that since I FIREd in January 2011. Unspent budget amounts just stay in the portfolio.
Staying on budget so far has been pretty easy for us. One reason is that it's similar to what we spent before FIRE. Another reason is that we always budget for the entire amount of our medical OOP deductible in our HDHP/HSA. It's about $11,000 and while some day I expect we'll actually spend it, so far we've never even spent 10% of that.
Our budget is pretty coarse, there is one big "monthly" spending item (about ½ our budget) for normal life expenses. We transfer money into the checking account for this (our monthly "paycheck") and we track it in Mint, but don't worry about the details as long as we stay under our limit. Additionally, there are seperate budget items for spending that is lumpy: property taxes, other taxes, health, auto, home, and umbrella insurance, travel, and a reserve category for normal stuff that comes up but it's a monthly expenses (e.g. last year we bought a new door, some furniture, and paid some unusual vet bills from this).
We were about 10% under budget this year and we don't treat that as "found money", it just stays in the portfolio and lets us spend a little more in the future.
We set our budget by figuring 4.5% of our financial assets at year end (this ignores our house) and average that with the last two years' budget amount. Basically a moving average. This smooths out our budget as the market blips up and down.
We have no pension and are not yet drawing SS.