1:50 is the 'metric equivalent' of 1/4" = 1'-0"
1:100 is the 'metirc equivalent' of 1/8" = 1'-0"
Alternatively,
1/4" = 1'-0" = 1:48
1/8" = 1'-0" = 1:96
You shouldn't have to do any more math, just make sure your units and views are set to imperial.
Cheers,
Link.