How do you type in a paperspace scale for the model viewport.

IN AUTOCAD If I want 1/4"=1'-0"  I type: zoom > 1/48xp > enter....but  in draft sight nothing changes. I can update the scale in the modify window. but I want a scale of 1"=50 feet and I cant select it (not on the list) or type in 1/600xp...what gives? if I type a decimal, I get some fucked up scale. as 1/600= .00166666666666666.....