Performance Planning is all about minimising risk. This could be described as avoiding disappointment. S o a good general approach might be stated as, Be pessimistic and you will avoid disappointment.
If we start our planning by looking at the distances available (TORA, ASDA, TODA) we reduce them by dividing by the safety factors.
If we start by assuming a perfectly flat, dry paved runway, we go through the performance charts to find out how much distance we will need. We then increase this by multiplying by the safety factors, and we then use these pesimistic figures to calculate our maximum take-off mass.