X / 1000 = Y

Where X is the value of the letter-spacing in Photoshop and Y is the value in em to use in CSS

Photoshop to em “formula”

X * S / 1000 = P

Where X is equal to the letter-spacing value in Photoshop, S is the font-size in pixels (which is equal to the value in point provided you’re working in 72dpi) and P is the resulted value in px to use in CSS

Photoshop to px “formula”