Hi Tony:

My 1936 Radio Handbook gives the following information:

"Grid-leak bias is quite flexible and more or less automatically adjusts
itself with any variation in RF excitation. The value of grid-leak resistor
is not particularly critical because the DC grid current usually decreases
as the grid-leak resistance increases, theeby keeping the product of the two
more or less constant for a given amount of RF excitation. Hence, the value
of the grid-leak resistance can vary from one-half to two times the optimum
value, a ration of four to one, without materially affecting the negative DC
bias voltages actually applied to the grid of the amplifier tube.

One of the disadvantages of grid-leak bias is that the bias voltage is
proportioonal to the RF excitation, thus precluding the use in grid
modulated or linear amlifiers, whose bias must be supplied from a
well-regulated voltage source so that the bias voltage is independent of
grid current."

So, I guess the answer is "use whatever value that makes the tube happy".

73, Colin K7FM