Sewer line drop per foot is set by the plumbing code, not by the contractor, and the table is short. Under the International Plumbing Code's Table 704.1, horizontal drainage pipe of 2 1/2 inches or less falls at least 1/4 inch per foot, pipe of 3 to 6 inches at least 1/8 inch per foot, and pipe of 8 inches or larger at least 1/16 inch per foot, with piping upstream of a grease interceptor held at 1/4 inch per foot whatever its size. The table is cited below from UpCodes. The rest is multiplying by the run, and checking the local adoption.
The table
Three rows. 2 1/2 inches or less: 1/4 inch per foot. 3 to 6 inches: 1/8 inch per foot. 8 inches or larger: 1/16 inch per foot. Those are minimums; the code also requires the pipe to be laid in uniform alignment at a uniform slope, so the fall is spread evenly along the run and not made up in one place.
The maths for a run
Total fall is the slope multiplied by the run in feet. A 4-inch sewer over a 40-foot run at 1/8 inch per foot needs 5 inches of fall from the building to the connection. Where the available fall is less, the pipe size or the route changes, not the slope. The pipe size tool on this site is the reference for the diameters the table is keyed to.
Too steep is also a problem
A slope well beyond the minimum on a small drain lets the liquid outrun the solids, which is why the trade talks of a maximum as well as a minimum. Where a run has more fall than the pipe needs, the excess is taken in a vertical drop with fittings rather than in a steep run. Check the adopted code for the local rule on that.
Check the adopting jurisdiction
The figures above are the model code as adopted, and states and cities adopt the IPC or the UPC with amendments. The UpCodes page cited below shows the table in a state adoption. The local code is the one the inspector reads, and the estimate should say which code the work is priced to.
Questions people ask about sewer line drop per foot
What slope for a 4-inch sewer line?
At least 1/8 inch per foot under Table 704.1; 1/4 inch per foot if it is upstream of a grease interceptor.
How do I measure the slope on site?
A level with a slope reading, or the rise over a measured run with a string line. The tool on this site covers pipe size, not the level; the arithmetic is above.
Is this the UPC figure too?
The UPC carries its own table; check the adopted code where the job is. The estimate names the code it is priced to.