You could probably strip the entire bottom 12 inches at least. It will only produce loose leafy buds down there. Should be a stable genetic from 420, so it should be able to handle it. I've done 2 heavy defoliation on my current autoflower grow and they are just fine. I think these plants are a lot more resilient than people give them credit for. I would still leave a fair bit of the leaves nearer the top of the canopy to harvest the light but that's about it. It is very true what they say about buds directly exposed, closest to the light. They just turn out better.