you just need to find the lowest point and the highest point of the area the heightmap covers on google maps and find the value from that.

That is exactly what i did, but still if the range from scale is 0-100% and my heightmap just using a range of 67% doenst this mean that my height map is 33% smaller than it should be? Hope you understand my problem

I think the lowest point is 0 (black)

and the hightest is 57 (white)

that means

my lowest point is around 12 (21% black)

and my highest is around 50 (88% white)

so my range is 38 instead of 57

Hope that helps to understand my problem