I've had a quick look around the internet and found the beer should be carbonated to about 2.5 volumes and on the Brewer's Friend website, found a priming calculator, which tells me that I need 3g of DME per 500ml of beer as per the image below.

I'm not quite sure how to adjust the level of priming though. To compound matters, I've cold crashed the beer to 4c and the calculator lists the following caveat regarding cold crashing:
"However, if the beer was cold crashed, or put through a diacetyl rest, or the temperature changed for some other reason... you will need to use your judgment to decide which temperature is most representative. During cold crashing, some of the CO2 in the head space will go back into the beer. If you cold crashed for a very long time this may represent a significant increase in dissolved CO2."
does anyone have a formula or suggestion as to what I need to do?