In Young's double slit arrangement, let $\beta$be the fringe width and let I_o be the intensity at the central bright fringe. At a distance x from the central bright fringe, the intensity will be 

Path difference = x (d /D ) where  d is the separation between two sources and D is the distance of screen from two sources. 
Phase difference
$\mathrm{\varphi }=\frac{2\mathrm{\pi }}{\mathrm{\lambda }}\times \left[\mathrm{x}\left(\frac{\mathrm{d}}{\mathrm{D}}\right)\right] \Rightarrow \frac{\phi }{2}=\frac{\mathrm{\pi x}}{\beta }$
Intensity at a distance x from the central bright fringe is given by I = $I_0 {\cos }^2 \frac{\phi }{2}=I_0 {\cos }^2 \frac{\mathrm{\pi x}}{\beta }$