Measure current through a stepper motor coil
That waveform that you have presented looks like what I'd expect for the current through one coil of a stepper motor in action. Nothing weird here.

If you know the voltage across Nfa and you want to find the current through Nfa which is the same current that is flowing through the coil that is in series with Nfa. Then all you have to do is divide the measured voltage by the resistance of Nfa. Do the same thing for Nfb. 

You do not want to divide by the resistance of the coil, or inductance. It has nothing to do with your measurement. You are measuring across Nfa/Nfb and Nfa/Nfb alone. 



So for clarity, in case my words are going in through one ear and out the other.

\$I_{coil(a)}=I_{Nfa}=\dfrac{V_{Nfa}}{R_{Nfa}}\$

\$I_{coil(b)}=I_{Nfb}=\dfrac{V_{Nfb}}{R_{Nfb}}\$

If you do not know the value of \$R_{Nfa}\$ or \$R_{Nfb}\$, then grab a multimeter and measure it.