Found something that might be useful to all of us (nice Article concerning power measurements):
Tak a look at this
Here are rough estimates of the wattage required for you to hold particular speeds outdoors:
- 25 kmph (15 mph) – 92 watts
- 30 kmph (18-19 mph) – 143 watts
- 35 kmph (21-22 mph) – 212 watts*
- 37.5 kmph (23-24 mph) – 254 watts
- 40 kmph (24-25 mph) – 301 watts
- 45 kmph (28 mph) – 415 watts
So if you’re unsure if your power numbers are accurate in Zwift, just find the speed above that you can hold outdoors on a flat ride going all-out for 20+ minutes. Then see if that wattage matches what you see in Zwift when you’re giving it everything you’ve got!
If you aren’t a trained athlete (let’s say you’re in your first year or two of serious cycling) and you are of average size, chances are very slim that your FTP is anywhere above 300 (for males). Chances are it’s close to 200, and maybe 250 if you have good genes or have been training for several months. Many riders will be significantly lower than 200 at this phase, and that’s OK too!
I know what kind of effort I have to put in to hold 35 km/h on my training bike in the summer (which is roughly 210 watts accordings to the above).
Compared to that, 210 watts (level 35 on 60 rpm) on the IC8 is a walk in the park, probably easy to hold > 300 on the IC8 even for a beginner 
→ not realistic.