Could one of our esteemed devs or resident smart arses 
please point me to a correct method / custom mix to suit an assymetric frame. Or even better a definitive method of calculating the correct mixer numbers.
Please !!! Ive scoured the web and there seems to be a lot of mixed ideas and frequently different results for the exact same frame lol.
Hi. Although the Discovery flights well with the standard mixer, it represents a classic example of quad copter requiring a custom mix.
Unfortunately I will not be much help until Sunday, because I'm away from home.
However, the far more important data that you can get in the meantime is to search the center of thrust
Don't take into account any marker on the frame and proceed experimentally.
There is a way you can get a double goal, find the center of thrust and make sure that it is aligned with the center of gravity.
You have to set to zero the Pitch Integral value in the inner loop section.
Then hover the copter in attitude mode.
You must move the weights on board up to stop the pitch drift (stable hover at Tx center stick).
Now, without moving anything, hang the TBS by a thread, at the point where it is perfectly balanced.
That's the point from which you can measure the xy coordinates of each motor and with which you can calculate a custom mix.