the A/R figure is kind of complicated to explain and i'm no expert but from my (limited) knowledge..

if you look at the where the housing begins, at it's inlet ( outlet for compressor housing) but where it's first at a 'cylinder shaped point' and measure the area at that point ( say you pushed a coin in as far as it could go, then measure the area of that coin ) now measure from the centre point of that coin to the centre point of the shaft, this is the 'R' (radius ) . divide this distance by the area of the coin and you get a your ratio;

so say the coin is 5cm in diameter, do piR2 to get area ( pi X radius squared ) coin = 19.635 cm
distance between centre of coin and shaft = 9cm
9 divided by 19.635 = A/R of 0.45

if you do this same process throughout the housing the ratio remains the same, because as the housing gets smaller, you are closer to the shaft

so A/R stands for 'Area of inlet cross section vs Radius

excuse the shoddy pic



if it resizes , the text at the top says area of inlet cross section and is pointing at the 'coin', the two thick lines are where the radius is measured from

that is MY understanding anyway, i accept i'm probably wrong in a few areas