BP changes with every minute of every day.
The monitors at your local pharmacy have sleeves that are meant for old ladies. My arms are 17" cold and I can't fit into the sleeve.
Ask the pharmacist, if the sleeve is too tight, you will NOT get an accurate reading.
Your doc is the best bet. Nobody should be running gear w/o supervision.
Some people say "be honest with your doctor about everything" ~ well, if you decide to do that, make sure he doesn't put anything in his records because there are situations that may arise later in life where all of your medical records might possibly be subpoenaed...
He doesn't have to write everything down to help you stay alive.