How is quasi peak calculated
Quasi-peak means 'not quite peak'.

It is intended to detect peaks that exceed a certain duration in RF with pulse amplitudes using a low pass filter, defined for EMC calibration testers and used to shape spectral curves for Audio Noise.  The weight is served to emphasize the peaks that cause most of the nuisance noises. There are other interpretations depending on the field of application.

I recall seeing this in the early 80's in a Paoli, PA Burroughs EMC test facility. Spike bursts of radiation usually got partially suppressed so extended bursts got detected. The RC time constant was probably in the &lt;1 millisecond range or &lt;5% duty cycle (est)

This expert also has a lot to say about it but still has questions on the formulae.  http://www.ieee.org/organizations/pubs/newsletters/emcs/summer01/pp.bronaugh.htm

Of course this has the acceptance criteria for quasi-peak, http://www.dbicorporation.com/ite.htm but not the calibration test or criteria for Quasi, which may be defined in this http://goo.gl/qxRR0

I think there is a quasi-analogical reason™ ;) why the specification for quasi is not readily available, so if you dont find one, I would use model it with 1st order LPF = T1 and decay to zero time constant T2.
My best guess is 100uS for T1 and 10mS for T2. 

Rationale for quasi-analogical is idiopathic. quasi-humour