For the individual items on my spreadsheet, I have a Pelouze postage scale that weighs up to 5 lbs. to the nearest 0.1 oz. or to the nearest gram. I got it on sale for about $25 back in 2005, but I notice that the current price at Office Depot is about double that. (Inflation at work!) In the interests of multiple use, I also find this scale very handy for cooking from European recipes or otherwise weighing food (weighing out the gram equivalent of 300 calories worth of nuts for my backpacking snacks, for instance). And, of course, it can be used for postage, which (by getting postage online) saves standing in line at the Post Office. The scale's accuracy gets checked every year at tax return time--I weigh the Oregon state return (which includes a photocopy of the Federal return so is right at 1 oz.) at home and at the Post Office (don't want it returned for insufficient postage on April 16!).
My bathroom scale is a Taylor digital scale, and appears accurate (i.e. I go straight home from the doctor's office and weigh myself on it) to the nearest 0.1 pound. That's certainly more than accurate enough for weighing the total pack! I weigh myself with and without the pack and do the necessary subtraction. Most people have bathroom scales anyway, so why not go for multiple use?
Last summer I went into a last-minute panic just before leaving for a trip because my total pack weight was almost 2 pounds less than what my spreadsheet said. I immediately unpacked and checked off every item, thinking I had forgotten something. It turned out that my food was the difference--I was using an average weight which turned out to be a bit high, and for 10 days it added up to the difference. Since I had leftover food every day, I certainly took more than enough!