Is there a standard definition of LPF stop band starting point in terms of dB attenuation
Sometimes people will define something different for the start of the transition band.  For years in the last century, Tektronix specified it's spectrum analyzer bandwidths at the 6dB point.  But it's rare (and a lot of Tek old timers insist it hurt sales)
There is no consensus.  It pretty much depends on what your system needs are.
It is decided by a systems engineer, who looks at both the consequences of signals leaking through the stop band, and the expected expense of having a lower stop band (70dB gets difficult with analog electronics, and lower stop bands require wider data paths in digital filters), and makes a decision.
There isn't a mathematical basis in the sense that you can start with a few basic axioms and reason your way to The Perfect Stop Band.  It's more a case of a knowledgeable systems person with a spreadsheet, juggling numbers and arguing with the implementation people on one side and the marketing people on the other.