Okay, I've solved this now, thankyou for your replies :)
It looks like it was struggling while it was trying to recognise what I wanted it to do, so I clarified it by adding in even more parenthesis.
Calc %w/w: IIf(Nz([%w/w],0)<>0,[%w/w],IIf(IsNumeric([Assay (mg/ml)]),(round(([Assay...