Order fo taking the derivative of a quotient
Mathematics   Wikipedia  
(high/low)' = (low * high' - high * low')/low2

 Mnemonic:
"lo-di-hi minus hi-di-lo all over lo squared", lo = low, hi = high, di = derivative

 Mnemonic:
"Low d high, Less high d low, Draw the line, O'er squared be low."