-3dB and the oscilloscope

[duck_arse]

I have a proper technical question for once. if I have 8 divs of signal displayed on my cro screen, does 8*0.707=5.65 divs indicate its -3dB point? and 4 divs is -6dB?

and now I have a single stage common emitter amplifier, with voltage divider bias, and 8 divs of signal fed into the blocking cap. if I then observe the signal at the base while I sweep the input signal, the point the display reaches 5.65 divs gives me the -3dB frequency, and R=1/2*pi*f*C then produces the amplifier input impedance, no?

so I did, and the impedance varied from 20k~30k, the frequency 232Hz~444Hz, depending on a 22nF cap or a 15nF cap. are they about the spread I should expect with this method, using eye and thumb? (R1//R2 results in 30k). will this method hold up for most cases, or did I get lucky? should I do the measure at the output instead?

and how does that series-pot-half-signal-point method work again?

[duck_arse]

I'm not actually replying to myself, I'm just bumping the thread to see if it gets an answer.

** bump **