I'm rephrasing my original post in the hopes it generates more replies than my original long-winded description. Ok, here goes my attempt at brevity.
Consider a symmetrical trapezoid with a top side measuring 200, bottom side measuring 800 and height of 450.
Placing this trapezoid on a grid with the top left being considered (0,0) and the bottom right being (800, 450) will give the horizontal center of the trapezoid at x:400.
Assuming the initial x value is a random number between with a range 300-400 and the initial y value is 1 I need a formula that can give the x value on a graph based on an increasing y value and the original x value. (Man, this is hard to explain succinctly.)
The initial x value also needs to influence the calculation as it is this number that directly affects the result. The angle should also increase as the value of the initial x moves away from x:400 (the horizontal center).
eg. When the initial xy = 400,1 then x should remain 400 as y increases (as 400 is the center and effectively 0 for my purposes. But as the initial x increases to a maximum of 500 then when y = 450 then x should equal 800. When initial x = 450 then when y = 450 x should equal 200.
Hope that makes sense. If not read my essay above.