About 13K at 25W should do it if it's a wirewound air-core resistor that isn't blocked from air flow. I sure wouldn't do that. Two OB2s and one OA2 in series are fine for the screens just as they are for AB1 operation. To switch to Class C where Eg2 can be much lower, just use a switch to tap down one OB2, leaving one OB2 and the OA2 in series for the regulated tap but all three tubes "firing." Don't use a pot, that's silly and defeats the purpose of regulation. You can do that, but I never bothered and never burned up a tube yet. My dual 4CX250B 2m amp was built by me in 1979 and still has the original tubes in it, working the same as they did in 1979. It's been through thousands of hours of transmitting over all those years. With proper LOADing you can adjust key-down screen current to be about zero. With keying or modulation, it can swing slightly positive or slightly negative. I use a zero-center meter in the power supply to see this, it indicates +/- 30mA but when the amp is tuned and LOADed properly it barely wiggles. Key factor is loading. A possible hazard is a feedline or antenna failure, which obviously will destroy the loading and can cause excessive screen current. If I feared that, that's when a fail-safe fault detector would be worthwhile; but it's never happened to me. When I tune up (almost same circuit as yours, just with different pipes and flapper vane tuning instead of the disks), I monitor RF output power, grid current and screen current only. I do have a plate current meter but never look at it. Increase drive slowly and peak plate tuning and loading for max output power, then increase drive more, re-peak adjustments while also keeping an eye on grid and screen current. With about 5-6W drive, output should hit 600-800W and final adjustment is LOADing only, crank it up for zero grid current and watch screen current -- it shouldn't be more than a few mA +/-. You'll see the LOAD control can actually zero the screen current. After going through this a few times, tuning up takes about five seconds. Unless you bump the controls or change frequency quite a lot (or change antennas), it won't need to be re-tuned at all.