I'm thinking of taking a ruler, from left edge measure to a point called X, then multiply by DPI value. That is X starting point.
Take ruler measure from top to Y, multiply by DPI value, that is Y starting point.
Example, 6 inches from left edge and 1 inch from top edge having DPI of 300, that would be X = 1800, Y = 300.
Of course, if you use something like A4, you probably should use PPM instead of DPI and use millimeters.
