I take tablets for my blood pressure and because a reading taken in a surgery is usually high (white coat syndrome) I always take along readings I have taken on my own machine at home ( £30 Boots). I usually take 3 at a time and note down the lowest one. Occasionally, I get three readings wildly apart, e.g. 120s, 130s and 140s. Why should my pressure jump about so much within two or three minutes?
BP can vary quite a bit even if a medic is taking it. I've had mine taken in a clinic and it's varied by 15 to 20 points in under a minute. That's why you should be wary of taking BP pills prescribed by the snake-oil merchants.
Omron instructions (the make apparently favoured by docs) say to wait 10 mins between measurements, I think to allow the cuff/tube/etc to empty themselves of residual air. Thus your readings might be taken too quickly.
Also, for at least a minute before and during the measuring, do absolutely NOTHING. Movement, yakking, etc, will distort your reading.
£30 is a bit cheap - Omron do one for £35 ish, but most of theirs are £60 plus.
My old GP alwsys chatted with me a while before taking my blood pressure. If I went for any other reason, he often suggested a quick BP check whilst I was there and calm! I miss that GP so much. He knew me well and cared for me for 27 years.
I doubt that a pharmacist sold BP machine would be at the same standard as those used by the doc. Maybe use the home one as a general guide to spot for continuous periods of unusually high, or low.
Omron are the best machines. Make sure you sit still, don't talk and stay relaxed between takes. One nurse at the surgery used to hold a conversation with me and my BP would rocket. I told her to shut up in the end. She laughed but she let me sit quietly for a couple of minutes and it went down to normal.
The literature that came with my Omron M3 Comforter advises:
No bathing, alcohol, caffeine, smoking, exercise, or eating for 30 minutes before taking a reading.
Rest for at least 5 minutes before taking a measurement.
Take the reading in a quiet pace.
Remove any tight fitting clothing around your arm.
It is automatic machine which has some feature to calculate blood randomly and its worse things that it can not compatible with our body mechanism. So It is need to think about other ways to control blood pressure and it should be natural path to over come.