The levels of THC and CBD can very greatly if you use two parents that have 20% of THC and CBD. Also in this small example some are 2:3, 3:5, 4:5 crosses, you may see 1:2 in a larger graph up to 20%. Then you’re going to have to grow a lot of plants to test to make sure you get what you’re looking for.
You would have to assume some testing was done to have perfect parents. With seed companies saying low THC or CBD and that the F1’s you receive are also stabilised.
I still think if you’re aiming for something specific it’s best to test, or get seeds that guarantee the ratio and levels you’re looking for.