Can you tell me based on your opinion what you think this 400 gram ball pythons gender is totally off of their tail
Nope. Ball pythons aren’t sexually dimorphic in that fashion. Eventually females get to a larger size but they don’t always exceed the size of a large male. With most Colubrids you can sex them visually, but it doesn’t work with pythons.
I’d recommend taking your snake to a trusted breeder or reptile/exotic vet to have it popped/probed to determine sex. Unfortunately you cannot tell sex base on tail, as @ballornothing stated.
I agree with the above 2 comments . Some snakes you can get some idea that way or with tail scale counts a bit, but not not ball pythons.
Any ball python I buy that I am not confident about due to the breeders experience, I probe to confirm male or female before purchase. Always and especially the UK equivalent of rehomed or crages list.