I did some thinking about your meter.
Your transformers are 1:12 turns.
There are two 50 ohm resistors. One for forward and one for reverse.
This meter does not read 'power' but current or voltage and you have to x^2 to get power.
The voltage across the 50 resistor is: 1V for 2.88 watts, 2V=11.52W, and 0.5V=0.72W. (notice the squared function) (12V on the coax, 2.88 watts, 50 ohm antenna, 1V on the output.)
Now to calibrate the meter which is not easy. When the output (across the 50 resistor) is 1 volt average the meter will see 1.414 volts because it is a peak meter not average. Take the meter, 1N34A, 0.01uf and the 25k pot (set to mid range) and apply 1.414 volts DC to the circuit. Use the pot to adjust the meter to read anywhere you want maybe 50%. (a well used battery will output about 1.4V) Mark this place on the meter 2.88watts. Now apply 2.818 V and later 0.707V DC and mark 11.5W and .72W.
These numbers are strange. You can work the math backwards. What DC voltage comes form 5W?, 4W? 3W? 2W? 1W?
Note the meter will not read linear because there is a 'squared' function and the diode makes the meter non-linear.
Please, some one, check my math.