I dont have the time to download and open your XF config , but heres the math to verify it :
example resolution: 1600x1200
100 dpi == 100 dots per inch,
x = 1600
1600 / 100 = 16 inch
16 / 0.0024606299 = 406.4 mm
(Res / DPI) /0.0024606299 == mm
or, the mm you want to get the dpi.
or, you could just measure the size of your screen, then select the mode that gives you "real" 100 dpi.