Find the maximum plate (anode) dissipation for Class A on the data sheet. For an EL34, this is 25 watts. Divide this by your plate to cathode voltage as measured from the plate (anode) to the cathode pins on the socket with the tube installed. You can also subtract your cathode voltage (as measured to ground) from your plate voltage (also measured to ground) to get your plate to cathode voltage.
For the purposes of illustration for this example, 25 watts / 340 v = 73.5mA. This would be your current draw to shoot for in SE Class A. Note when using a tube that draws more current than the 6V6, like an EL34 you plate voltage may go down depending on how efficient your PT is. If you want to play it safe, you could bias your amp at 90% of 73.5mA or 66mA. To increase your mA draw, decrease the value of the cathode bias resistor. If your amp is drawing too much current, increase the value of the cathode bias resistor. You may want to get a few 5W, cement resistors in the 220 ohm to 680 ohm standard range to experiment with.
Once you install your resistor, you have to check the bias again along with your plate voltages, cathode voltages and current draw, then recalculate. If your current draw is is within the 90%-100% you're done.