If you're looking into reducing BP, the strongest correlation is with body mass. If you have the margin to lose weight, that should be the primary approach. The correlation is roughly 1mmHg per Kg in systolic BP. Diastolic BP drops less per Kg, but is also affected.

There is also a strong correlation with height, but I don't advise losing height :-)

For many people that is true. I have significantly reduced my BMI and my BP actually went up. I do Keto+IF+Berberine and do 2 day FMD, soon to try 5 day FMD. My BP issues are likely due to calcification and inflammation related to metabolic syndrome. I have a blog where I am tracking this if you are curious. [1]

[1] - https://ohblog.org/Metabolic-Syndrome-Hypertension-My-Journe...

If I'm already at a healthy but low BMI, should I increase my body mass so that in case of high blood pressure, I have weight to lose?

You could look into building muscle mass. I would avoid eating junk food and drinking beer until you get a belly, for instance.

A bit of both would probably be okay, though.

What dosage do you use? Over what period have you seen BP reduce?

I started with 4000 IU's D3 + 200 mcg K2-MK7 per fatty meal or snack. I am now up to 12000 IU D3 + 200mcg K2 per fatty meal or snack. This took about a year. My goal is to reach 40,000 IU's per fatty meal, but I can't do that yet, as too much calcium is released into the blood stream and causes muscle pain.