I have now no problem with this but as you can imagine, I still have 1 problem ^^, certainly due to my approximate understanding of English I am unable to find a formula for point to point substraction. Is this also possible?
there is no special formula for point subtraction as far as i know. instead the P-Q is simply defined as P+(-Q) (same as addition) and -Q or negative of a point is defined as negating its y coordinate. or in other words -Q(x,y) = Q(x,-y) and since we don't use negative numbers in modular arithmetic -y becomes P-y where P is curve's prime.