Probably a stupid bias question

[Snake5150]

Jtm 45

As I decrease the grid bias voltage the voltage across the cathode goes up as it should but simultaneously starts to bring down the plate voltage.

If I now re calc my needed cathode drop it will be more cuz the plate is lower.

So were do I being the calculated numbers from. All the way cold bias and thus most plate voltage?

My plate voltage has dropped from 446 to 430 ish to achieve 35 mv of drop across the cathode. My original math said about 38-39 mv

[AlNewman]

It's not an exact science, for the most part when we predict what the voltages and current will be based on datasheets, there will always be some variables.
When you draw more current across the transformer, the voltage will go down.  Looks like you're within 10% of what you predicted, so you're doing good.

[stratomaster]

The best way to look at it is you're trying to reach a certain power target (idle dissipation), not a certain current or voltage number.  When you rotate your bias pot it is affecting the grid voltage directly which in turn affects both cathode current and plate voltage.  This is because you're altering the idle power dissipation of the tube.  Rotate one way and the power goes up, the other and the power goes down, regardless of what is happening to the cathode current or plate voltage. 

[Latole]

The best way to look at it is you're trying to reach a certain power target (idle dissipation), not a certain current or voltage number.

100 % right .
I bias no more than 60% power dissipation.
70 % often see may harm tubes at high power