duty cycle is the ratio of the non-zero component of the pulse to the total time of a single pulse. Freq. is Hz..i.e. osc / second...
You can run a 10Hz signal and have 50% duty - i.e. square wave. If you adjust the duty cycle would can still run 10Hz but have a duty of say 70%....longer on pulses...however the freq can remain the same...
Hence as you have suggested, once you have found the op. freq. of our soladoid then there is not need to adjust freq..just duty and I assume thats what most major products are doing...adjusting pulse width at a constant frequency.
As for the AVCR...it displays duty at a %, maxx 100...so I assume it dispaying the actual duty..