I'll echo the Caliber IV recommendations. I own six of them, plus an older Caliber III. None were more than two points off at either 75% or 65% (both tests using Bovedas), and the Caliber IV is adjustble up to six points plus or minus. About $25 most places, but I've seen them for $20 on occasion. They use the common CR2032 button cells (about the size of a quarter), and should last six months to a year or longer; if your reading seems to be off, it's probably a battery near the end of its useful like. (I buy CR2032s in five-packs and keep spares on hand.)