Uusing the ITU-R BT.601-4 doesn't really change the relationship between PAL & NTSC (both 4:3 and 16:9), if you input letterbox PAL values you'll get virtually the same results for PAL/NTSC 4:3/16:9 outputs as with the previous aspect ratio calculations, it's the relationship between 4:3/16:9 and 1:1 resolutions that change, you get a more true PC aspect ratio when converting PAL/NTSC source to PC.

The checkbox is an option I'm intending to impliment in a future revision, you can still access the first version here if you need it: http://www.haku.co.uk/ARCT1.0.html