Free rides

Hello,

I am doing several training sessions from Zwift and in the latest, I am doing I am asked to do free rides, like with 1 min, 4 min, 8 min durations.

I find it really hard to determine the amount of Watts I will try and hold during such a period. Even in the one minute (they told me to go for it) I went for it, but I totally blew up after 45 seconds and my legs were rubber.

Is there a broad guideline where time is compared to your % FTP? Like if FTP is 95% for 20 min then 5 minutes try and go for 125% or something?

Help is appreciated.