Transmit power question

I am looking to obtain 24dBm (251mW) transmit power on a UniFi AC Pro radio. The UniFi itself has 3dBi internal antennae.

In order to achieve this, do I set option txpower 24?

Or do I need to set option txpower 21 to factor in the 3dBi antennae, to provide an effective txpower of 24?