I was confused by the Area Factor bit as well. So I did a little formulating.
Starting with: Area = F * hypotenuse^2
Replace Area: 1/2 Base * Height = F * hypotenuse^2
Rearrange: (Base * Height) / (2 * hypotenuse^2) = F
There is your area factor.
Making this useful, if you only knew you had two similar triangles with hypotenuses 3 and 4 respectively (as in the example) simply go back to Pythagoreas and solve for the missing hypotenuse of the combined triangles, knowing of course that the respective smaller hypotenuses become the sides of the larger triangle.
As Kalid said the Area Factor doesn't really matter, but I still wanted to find a formula for it...voila.