Did you know that there is engineering methods that can be used to increase safety factors?
If the base of a ramp or platform is at least 1.2 times the height of a ramp then it increases stability, thus making it much less likely to tip over (As you see with circus platforms for elephants)
If the height is 12" (300mm near enough) then your ramp should have a base of at least 14" (360mm) and obviously if you taper the platform height smaller than your footing then it will also increase stability.
Hence the reason why my platforms (at this stage) should be around 12" (300mm) high, 12" (300mm) wide by 17.75" (450mm) long at the top and 13.75" (350mm) wide by 19.7" (500mm) long at the bottom.
Constructing with the taper means that the weight has to travel further past the center of gravity before it can be displaced, and the effort to make the weight travel past the centre of gravity is a lot harder to make happen.