Well no-one has yet explained what W+B calculations are all about. So let's try and put the OP in the picture..
When doing Moments at college, it was explained to me that, If you take any point on a body at rest, then the sum of the clockwise moments will equal the sum of the anti-clockwise moments. The Moments are defined as being the weights times their distance from that point.
To make thing simpler, you could take moments around the C/G, then the moment of the C/G would be zero, and would simplify the maths.
It doesn't matter where you put your datum point, anticlockwise will always equal clockwise. For the Cessna the datum point is somewhere near the back of the bonnet, but the calculations would work just the same even if it was at the tip of the prop-spinner. ( looking at the worked example, it is 39 inches in front of the seats, and the centre of mass of the oil is 14.7 inches in front of it.)
Best of luck, and study the POH for your particular aircraft.
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