How did you measure QRS complex?

Use INA OP, and detect algorithm: https://github.com/GCY/wxECGAnalyzer

Just looked up Max3010x =>  pulse oximeter to pick up systolic peak.  Amazing -   BP measurement without any actual pressure measurement.   Must admit I am a bit sceptical of the strength of this correlation with BP.  Just have to measure and see I suppose.  Even so,   the PWV might turn out to be a very valuable parameter in its own right

 right, so PWV-BP method are for investigational use only.

Brilliant idea to use QRS as indicator of the start of the wave at the heart.  The relationship between BP and PWT was new to me, so I followed your references - thanks.  It seems that the relationship is indirect via a change in arterial stiffness with BP and and increase in arterial blood volume via BP.  I assume arterial blood volume mostly depends on arterial geometry.  Arterial stiffness is probably an independent variable that may well have a clinical significance possibly as much as BP itself.   As a measure of BP, for instance as a replacement for ambulatory BP it seems to me that there are three factors:  1. A requirement for calibration.  2. How stable that calibration is (hours days weeks?).  3 Actual performance- ie how closely the measured value correlates with BP measurements.

I never knew that was possible! I'd love to learn more about the theory behind this method. Any references?

The theory based on pulse wave velocity : https://en.wikipedia.org/wiki/Pulse_wave_velocity

In general, pulse wave  arrival time and blood pressure is nonlinear relationship.

https://www.zdnet.com/article/blood-pressure-and-the-samsung-galaxy-watch-active-2-regular-monitoring-is-key-to-diagnosis/