Calibrating transmit power in SMARTTX POWER mode

Is there a way to calibrate the transmit power at 41.3dBm/MHz without dedicated measuring devices?

The general answer would be No.

However, if your application allows slightly reduced TX power, you can characterize your product, say use a small batch of 1000 devices, and set the median TX power to the level of 41.3dBm/Mhz minus 3sigma. The worst case for you would be >5sigma less TX power on less than 2.5% of devices.