Calculating load resistance?

[jonnycat]

In calculating the load resistance for a particular tube, do I just divide the plate voltage by the combined plate and screen currents to come up with the answer?

Also, when measuring the plate voltage, when using a cathode bias scheme, I measure from the plate to the cathode (and not ground), right?

Sorry if these are really stupid questions, but I have no formal training in this.

Thanks,

Jon