Determining a loadline graphically with tube data

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Hi guys, I am teaching myself how to determine a loadline to find out the primary impedance of OPT I should use for certain tubes. I am not quite sure if I am doing this right, but this is what I have so far. I used 1626 tube for my calculations:
- Pa max = 5W. I used P=IV to plot several points going from 10 to 300 volt on Ia vs. Va graph. I did regression on Excel to get a equation of line (power) that turned out to be y=5000/x.
- I wanted to get an equation of tangent line to this curve, so I differentiated f(y)=5000/x to get f'(y)=-5000/x2. Plugged in x=55 (tangent point) to get a slope as m=-1.65.
-Then plugged in (x,y)=(55,90.9) and m=-1.65 in y-yo=m(x-xo) to get an equation of tangent line, which was y=-1.65x+181.66.
- Got X-intercept w/ y=0 at x=110 volt, and Y-intercept w/ x=0 at y=181.66 mA.
- So, I have my loadline going from 181.66 mA to 110 volt. So, it corresponds to 110V/0.181 A=608 ohm using Ohm's law.
I don't know why I am getting my rL on 1626 so low. In practice, Darling amps use 5k primary impedance for OPT.
Any help on this will be appreciated.
-Min