Reducing 12v DC to 0.5V DC no more than 1A draw

[AdCNC]

Is it critical for the value of the zener diode as I can only get my hands on a 2.7v jobbie today

[m_c]

No, but a higher voltage zener will mean the potentiometer will be working a bit harder.

[FatFreddie]

Is it critical for the value of the zener diode as I can only get my hands on a 2.7v jobbie today

Not really, your output voltage will be 0 - 2.7v rather than 0 - 1.5 so trimming it will be slightly more fiddly.

I'd set it to zero and connect it up and then bring it up to the voltage you want - this will negate any effects due to the load the ECU puts on the potential divider.