What I do is create an increment for a centimeter (in your case, inch) for height and one for width.
#CM_HEIGHT or #IN_HEIGHT = (customer back waist length divided by the back waist length used in the original pattern)
#CM_WIDTH or #IN_WIDTH = (customer chest circumference divided by the chest circumference used in the original pattern)
From 16 square out 1*#CM_WIDTH (this particular adjustment affects the fit around the torso, so use #CM_WIDTH)
These formulas scale the number values up or down relative to the customer's major measurement for height and width. This can prevent having to introduce a conditional statement like (bust_circ > 50? 1.5 : 0.75)
Sometimes I calculate the percentage in the point's formula instead of in Increments if the formula is used only once. So the formula might look like 1.08*bust_circ. For a 38" chest, this adds about 3" to the chest girth (3.04").