How to judge auto takeoff % for maidens

How do y’all judge the auto takeoff % because I’ve launched three different birds (different models) recently and at 100% throttle they all reacted very differently for their maiden, obviously due to the nature of 3D printed aircraft each bird will be different build to build. I figure it’s just like a guess the thrust in Kg from the prop to motor to battery size but just wondering if anyone has an easier, more sure fire way.