If the end points are A(a,b) and C(c,d) and you want the point 3/4 of the way from A to C
the co-ordinates are (.25a+.75c, .25b+.75d).
This is an application of the side-splitter theorem from plane geometry and it would certainly be worth your time to go show your self that proof. And if you can't prove it, make a comment to that effect and I will try to show you how to do it.