Thanks Zhango, I think a consensus is emerging...
As I don't have access to information regarding a spring guage, that method is out.
As I don't trust the twist method (a strong person will get 45 deg where a weak person wouldnt be able to move it) that method is out.
I haven't access to a decent sonic frequency counter, and the use of a mobile phone or a laptop with some downoaded program sounds too prone to improper use or inaccuracy with a big question over calibration, so that method is out.
Therefore I'll go with the allen key method and take Zambo's and others advice with the 5mm H and 6mm V measurement.
(has anyone ever measured the sonic frequency after setting with an allen key?)