My wife takes my BP manually, but I want to start taking multiple readings in a day. I once had the "best", most expensive unit (or two) that Walgreens sold, but the readings were off. Even in my doctor's office, they've taken my BP electronically then immediately taken it manually, and the readings have been off as much as 20 points on systolic pressure. But I'm thinking that with frequent calibration with my wife's manual readings that we can extrapolate useful info? Can you recommend a good unit?