What value grid leak resistor?
With the linestage, I am using grid chokes (actually small line transformers wired with the windings in series.) With the power amp, I am using 18K but that is in parallel with a very low DC resistance coming from a transformer volume control so the effective DC resistance is only a few ohms here. I ran into a situation where 18K was too low a value for a certain preamp to feed, so I used something like 100K for that setup. The lower the better, but it depends what you are feeding it with how low you can go. A grid choke is ideal.
How did you calculate the effective load the plate choke presents?
Well, I didn't actually: first I had on hand a Lundahl interstage transformer and I tried it and it worked well. Then a friend was ordering from Lundahl a bunch of stuff and I piggybacked on his order and got the corresponding plate chokes.
The rp of the 6C45P is so low that you don't need a tremendously high inductance to load it: almost anything meant as a plate choke should do fine, I think.
I tend to use what I have on hand, at least at first. I really didn't expect the line stage to work, it's a bit extreme (high mu/huge step-down), so I just slapped it together. Then it turned out I liked it, so it just sort of stayed slapped.