Most avanced HRV-biofeedback device for Windows PC is Balance Manager.  This  instrument, developed in Korea has a the simple  and effective breath pacer, with personal setting and shoping clear graphics to represent the state of the autonomic nervous system. Sense en simplicity!  For sale (189 euro) in the webshop or with email to National Centre Stressmanagement:
In Balance Manager newest developments in research with HeartRateVariability biofeedback are integrated: private setting of the personal resonance frequency.
Longer exhalation than inhalation is also important in stressmanagement and important to avoid hyperventilation. Balance Manager creates feedback in emotion regulation processing and with this feedback (psycho)therapists can intervene more effective and more safe.