“Every inhalation creates two heart beats during which the Qi moves three cun. 
Every exhalation moves Qi another three cun and creates another two beats. 
So a Full breath has four beats and moves the Qi six cun. 
The rate of this movement is said to be normal when there are four pulse beats per breath.”
Mai Jing 

Classically, the rate of the patient’s pulse was measured against the breath of the practitioner because practitioners were considered to be at a high standard of cultivation.  These days, however, as we practitioners are perhaps not adequately cultivating ourselves, the pulse rate is measured against the patient’s own breath.   Interestingly, the word Rapid also translates as arithmetic, implying that one should count the pulses against the breath.  

The clock is immaterial and not used at all.  It can hinder the diagnosis.  The rate must be read as a component within the individual’s system, not measured by an artificial or external standard.  We are interested in the relationship between the breath and the pulse in the individual.

Some positions can naturally be slightly more rapid than others.  The cun is more likely to be slightly rapid than the chi position because the Qi examined in the cun is more lively than that in the chi.