It can be either, though often it is the bottom.
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scale factor
It is the side of an isolateral figure.
Multiply the length of the side by the length of the top or bottom.
Measure the length of a side in the first figure = L1. Measure the length of the corresponding side in the second figure = L2. Then, provided L1 and L2 are in the same units, the relevant ratio is L1/L2.
You divide the length of a side of the first figure by the length of the line in the same relative position in the second figure.