Given that I have found a tx that is for me, what would be the maths to find out how much I have received?
It's impossible to reverse the Pedersen Commitment if you don't know the mask.
The commitment is
amount*H + mask*G, so if you were told the secret mask, it would be possible to brute force all likely amount values to discover the amount.
But then if the mask can be securely communicated to you, so can the amount. So this is what Monero does. It encrypts the amount using the transaction shared secret, and bundles that encrypted amount as part of the transaction.
The decryption math is:
decrypted amount = encrypted amount - Hs(Hs(8aR||i)) where
a is the private view key,
R is the tx public key and
i is the index of the output whose amount we're decrypting.