Prism to cut

I'm trying to calculate prism to cut with these formulas but they need a certain sign convention.

P_V=x*Sphere + Cylinder*sin(Φ)*(x*sin(Φ) + y*cos(Φ))
P_H=y*Sphere + Cylinder*cos(Φ)*(x*sin(Φ) + y*cos(Φ))
P_r=sqrt(P_v^2 + P_H^2)
x = horizontal decentration
y = vertical decentration

Can anyone break down the sign convention used. It's a little counter-intuitive to me.

Thanks,
Rob

P.S. This is not homework

Prism to cut