Like others have said, it really depends on the company you are with. With some you'll have almost insane deadhead percentages, like if you only have outbound freight and are always deadheading back to get another load. That's gonna be at least 50%, assuming that you occasionally go home or drive someplace besides front point A to point B and back again.
If you are with an expedite company with a large presence, like Panther or FECC, you should expect somewhere around 25% to 30% deadhead, which includes paid and unpaid deadhead, and any unaccounted for miles (depending on the terminology you choose to use). I use my van for personal stuff, not a lot, but it's there, and it's working out that just about a third of all of the miles I put on the van are unloaded miles.
Some people will deadhead at the drop of a hat, immediately tossing out the deadhead miles as a factor because they are heading for another load, and loaded miles is all that matters. That's actually an attitude that many expediting companies would like you to adopt, and it's one that I heard pounded hard, albeit subtly, at an orientation recently. And it's working, to a degree. I've heard drivers who have come out of that orientation, new and old timers alike, say things like they never worry about deadhead miles, that they only look at the loaded miles.
These are the drivers I worry about, actually (van drivers, mostly, obviously). They are the ones who will deadhead without a second thought, and will take a load regardless of their state of rest and sleep. They trade sleep for deadhead hours, just to get a load. These are drivers who are working much harder for their money, but with no more money to show for it, really.
I talked with a driver 2 or 3 weeks ago who went on and on about not worrying about deadhead miles, that loaded was what mattered. We did some figuring and it opened his eyes. Over the last 8 weeks he's average 1300 miles per week loaded, while at the same time averaging a little under 800 miles a week as unpaid miles. That's 60%. He rarely uses his van for personal stuff, but he does go home every weekend. He no longer wonders why he doesn't seem to make as much money as he thinks he should.
I try to look at the overall big picture when it comes to deadhead, rather than matching up deadhead with a particular load. I'll look at a load and if it's significantly more than 25% or 30% deadhead, I want to determine not if that particular deadhead is profitable on that particular load, but will it get me to a place that will get me another load, or will it get me to a place that's just gonna require even more deadhead to get to the next load.
I also figure deadhead as all miles driven from last delivery to the next pickup (much to the chagrin of many a Panther dispatcher hehe). I've just driven 150 miles from my last delivery to where I'm sitting now. If my next load offer is for 150 miles deadhead for 500 loaded, in reality it's 300 miles for 500 loaded, which is 60% deadhead, but if it takes me to a place where my next load is likely to be little or no deadhead, then it all evens out in the long run.
I'm running 25% to 30% deadhead, and I'm averaging 2300 miles a week. My actual loaded miles per week when I'm in service is a little higher than that, tho. When I figure my weekly average miles, I include days and weeks where I'm out of service in the equation. My annual goal is 120,000 miles, and that, divided by 52 weeks, whether I'm working or not, comes out to 2308 miles per week. If I take 2 or 4 weeks off a year, vacations, repairs, whatever, that means that when I'm actually working I need to get between 2400-2500 miles a week to make my goal. I've had 4000 mile weeks and I've had 800 mile weeks, but so far I've always been within 10% of my goal every year. And usually the 10% difference, above or below, can be traced back to in increase or decrease in my deadhead miles.
They key is to keep your availability high, be willing to sit for a few days instead of doing a long deadhead when in areas where the next load is likely to be a really good one (like Laredo), but don't sit for a few days in an area where your next load is likely to be a short one (like Nashville). Use deadhead to your advantage, instead of either dismissing it entirely or viewing it as a necessary evil.
OK, so I can ramble. But maybe someone can get some use out of my ramblings.