Apologies, I just noticed I have forgotten to reply to you with the issue, Tom.
I've stuck to the suggested values and my initial set of power measuring results are within 0.003% of a yokogawa WT210, which seems pretty good. I'll have to dig a bit deeper to test higher power conditions, non PF=1, temperature swings and so on. But for residential indoor use, these results are promising.
Thanks for trying to help my friend,
Best wishes!