The Church expects pastors to "teach and model generous Christian giving with a focus on tithing as God’s standard of giving."
In addition, each annual (regional) conference, as part of its responsibilities in the area of stewardship, is to "educate the local church that tithing is the minimum goal of giving in The United Methodist Church."
The Church says that tithing is "the minimum goal of giving....", but does not specify net or gross. Some people tithe out of their gross income. Many tithe from their adjusted gross income and many tithe out of the after tax income. The important thing is to begin to tithe. God can always work on the details later.
Since the tithe was figured in so many different ways in the Hebrew Bible, Rev. Clif Christopher and Rev. Herb Mather took the stance in their book Holy Smoke! Whatever Happened to Tithing? that the tithe is a benchmark. Just as people and institutions need all sorts of benchmarks to measure growth, the tithe can be one (of many) benchmarks of spiritual growth. If a person accepts that possibility, then they recommend starting with a tithe on the net and then moving beyond to setting a benchmark of tithing on the gross.
Read more about tithing.
Tithing: Law of God or Gift of God? (Part I)
Tithing: Law of God or Gift of God? (Part II)
Five Reasons to Tithe
Tithing: A Benchmark for Giving
On the Pilgrim's Way: Christian Stewardship and the Tithe