I’ve just moved from an older Emonbase to the new EmonP monitoring the main solar array. Also running and connected to it is an EmonTxshield which measures the output from a second solar array. This gives a voltage of approx 238V. The EmonPi is consistantly approx 11v lower. Using a volt meter shows that the Emontxshield is correct.
Am I correct that the EmonPi voltage will render the power calcs. incorrect?
How do I adjust the calibration on the emonPi?

That is correct. You have obviously calibrated your Shield correctly and compensated for the manufacturing tolerances of the components used. You can do the same thing with your emonPi, but the process is somewhat more tricky, because you have to use the Pi itself to load the modified sketch into the Atmel 328P via the serial connection. Far easier is to calibrate the data as it is received in emonHub, and you can do that in emonhub.conf, which is accessible via the browser interface.
In the section for Node 5, which is the Pi’s own readings, you need to alter the scale factor (“scales = …”) from 1.0 for each power, or 0.01 in the case of the voltage. Note that adjusting the voltage scale factor here does not alter the power value.