y = AsinB(x-C) +D.
I know that A (amplitude) can be found by getting (max-min )/2, B = 2pi / current period, and D = average of max and min y.
But how do you find C, once you've found those 3?
C is the horizontal shift of sin(B[x-c]) compared to sin(Bx)
you can calculate it most easily at the origin because $\displaystyle \sin(Bx)$ passes through (0,0) for all B. The value of $\displaystyle \sin(B[x-c])$ can be found by where it intersects the x axis (ie: y=0). For example if C=45 degrees then $\displaystyle \sin(Bx-C)$ will be shifted to the right by 45 degrees compared to $\displaystyle \sin(Bx)$
Post 2 shows the mathematical representation
Edit: In case a picture helps: