There are two things you should do before calibrating in order to ensure accurate power readings.

First: Inflate Your Tire

Tacx recommends 7-8 bar (~100-120psi). To keep things simple I try to keep my tires at 110psi, which is what I ride at in the real world.

Why does it matter?

Your Vortex will calibrate accurately at any reasonable tire pressure, but unless you want to calibrate before every single ride you’ll want to decide on a consistent tire pressure to maintain. Otherwise, if you calibrate at one pressure, then hop on and ride at another pressure days later, your power readings will be off.

Second: Warm Up Your Tire

Ride for at least 3 minutes at a decent power level so the rubber is warm. If your tire is especially cold you’ll want to go longer. I recommend a 10-minute warmup.

Why does it matter?

As your tire warms up it becomes more pliable, effectively reducing the tension of the tire against the roller. In terms of power numbers, what this means is if you calibrate with a cold tire, the Vortex’s power numbers will be significantly higher than normal once the tire warms up.

(I experienced this myself accidentally in a race once: my numbers were ~60 watts higher than they should have been, because I calibrated on a cold tire! For a while I thought I was just having a really good day, but when Zwift wanted to update my FTP to 60 watts higher than it had previously been I knew something was afoot.)