IMO, without a powermeter on your bike, you can't get an accurate wattage for the speeds. I have the US version and my power can vary 20+ watts at the same speed depending on tire pressure and tires.
Yes, you are right. But for me the accuracy in range +/- 15 Watts will be OK. Only that I'm interested in is the tested data like: XXkg rider weight, X bars tyre pressure, XX km/h speed, XXX watts power
I hate to say it, but this just won't work.
If his "power" is 20+W off on any given day, what's he gonna say and how will you turn that into 15W off? You will get a close enough number when you are not pedaling at all
, but it will get worse the harder you go.
You are really better off not using power than just guessing. My advice is: Don't even look at level and speed. Use a HR monitor or go by feel alone.