Dave,
A few things:
1) The attenuator we sell is only rated for 2 watts. You would need a 10-20 watt attenuator.
2) Typically a directional coupler is used to monitor transmit power. This way, you only lose a small amount of power and you can monitor it while in use. Depending on the “coupling loss (typically 20 dB) you may want additional attenuation. I would recommend aiming for between -10 dBm and 0 dBm to make decent measurements.