http://www.sportys.com/acb/showdetl.cfm?DID=19&Product_ID=1199&CATID=171
I've had this little thing for 20 years or so, and I wouldn't operate out of any marginal strip without it, unless I was doing that on a daily basis. It's great for any new strip, any overgrown, muddy or otherwise iffy takeoff, once you get it calibrated for your plane. Once calibrated at whatever density altitude is convenient, by measuring lift-off distances, it will be real close for any conceivable runway situation. Best $23.00 I EVER spent in backcountry preparedness!
I have mentioned it before, but it was more than a year ago, so all you newer guys should check it out. It's been around for probably 50 years and is as simple as a brick. (but don't leave out in the rain.....don't ask me how I know this!
Your AFM will never tell you what to expect in a downhill, downwind, 110% of gross take-off at 80 degrees at 7500ft density with long grass, but this thing will, and I can verify that if properly calibrated and properly set, it will be VERY accurate. It has saved me from an almost certain take-off crash that looked like a sure thing.
Y'all be safe up there.
Rocky