1/4 " per ft is average
Vertically it is unrestricted
It really depends on your local building code, but 2% should be the minimum so that your pipe will be self-cleaning.
1/8" per foot
.25 per inch per foot
I would use 3 inch for that.
1/8" per foot.
Yes. A reduction fitting can be used.
A 3 or 4 inch pipe sticking straight up out of the ground? This is the clean out for the sewer line. If sewage is coming out of it, this means the sewer line is clogged or collapsed from there to the main city sewer. If you have a septic tank, the tank is full and needs to be pumped out.
For proper fall. Pipe should slope 1" for every 8'.
1/16 PER IN PER FT. But the Fixture unit value decreases to only 1440 as opposed to 7300 at 1/2 Pitch.
In Tulsa Oklahoma you can, but i would still use 4 in.