As for when to check your blood pressure, the most important thing is to do it consistently the same time of the day (ask the doctor which time he prefers and also what time in relation to taking your medication). The following article has a lot of good information for someone just starting to monitor their blood pressure: https://www.drugs.com/cg/how-to-take-a-blood-pressure.html
The new guidelines stem from the 2017 results of the Systolic Blood Pressure Intervention Trial (SPRINT), which studied more than 9,000 adults ages 50 and older who had systolic blood pressure (the top number in a reading) of 130 mm Hg or higher and at least one risk factor for cardiovascular disease. The study's aim was to find out whether treating blood pressure to lower the systolic number to 120 mm Hg or less was superior to the standard target of 140 mm Hg or less. The results found that targeting a systolic pressure of no more than 120 mm Hg reduced the chance of heart attacks, heart failure, or stroke over a three-year period.

Last year, new guidelines from the American Heart Association, the American College of Cardiology, and nine other health organizations lowered the numbers for the diagnosis of hypertension (high blood pressure) to 130/80 millimeters of mercury (mm Hg) and higher for all adults. The previous guidelines set the threshold at 140/90 mm Hg for people younger than age 65 and 150/80 mm Hg for those ages 65 and older.


Your total blood pressure reading is determined by measuring your systolic and diastolic blood pressures. Systolic blood pressure, the top number, measures the force your heart exerts on the walls of your arteries each time it beats. Diastolic blood pressure, the bottom number, measures the force your heart exerts on the walls of your arteries in between beats. 
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