The traditional slope is 1/4" drop per foot of run, which
basically equates to the 1 in 50 slope you mentioned.

Well yeah, all you have to do then is hold a 48" level with one end on
the pipe and level the bubble. If there's at least 1" of daylight on
the other end, you've got plenty of slope.

Many plumbing runs are less than 48", or are in cramped areas, which is why
I use a 24" level instead. I also have a small 8" level I use in really
tight areas.

Also, when you're holding the pipe in one hand, and adjusting the hanger in
the other, it's difficult to hold a level and look for daylight underneath.
Even if the pipe is supported on one end, you still need the second hand to
attach the hanger when the proper slope is achieved.

By taping a 1/2" plywood shim to the end of my 2' level, I can simply set
the level on top of the pipe and still have both hands available for
positioning and anchoring everything.

Anthony