1 way of finding out the Twips/Pixel is
[ Screen.TwipsPerPixelX ]
[ Screen.TwipsPerPixelY ]
as ICE correctly pointed out.

The Twips/Pixel changes depending on screen resolution NOT font size.

There are always 12000 X 9000 Twips on a screen.

Therefore at a resolution of 800 X 600 there are always

12000/800 = 15 Twips/Pixel
or
9000/600 = 15 Twips/Pixel

Work out the resolution you are working at and see that at different resolutions there are different numbers of Twips/Pixel.

Hope this helps,

Steve.