Andrew,
I have checked my speed on my BN with a digital readout through my computer using a program that is free called iSpectrum. What I do is play test frequencies and checked the Waterfall plot and it is spot on. My biggest factor is the belt. They need to be replaced when they get old otherwise you get a sling effect. But this has been by ear. I have excellent speed stability which I can watch on a screen. I don't use a strobe but instead the computer to set my speed. Seems to match the strobe though. Once set it stays put. I have checked it often and my speed stays steady.
I have checked my speed on my BN with a digital readout through my computer using a program that is free called iSpectrum. What I do is play test frequencies and checked the Waterfall plot and it is spot on. My biggest factor is the belt. They need to be replaced when they get old otherwise you get a sling effect. But this has been by ear. I have excellent speed stability which I can watch on a screen. I don't use a strobe but instead the computer to set my speed. Seems to match the strobe though. Once set it stays put. I have checked it often and my speed stays steady.