Most of the folks I have talked to find that they are getting good results with loads that yield velocities in the 1300 fps range. Try starting with loads that will yield this velocity range. This may be due to tradition, or other factors. A lot of folks have excellent luck with FFFg in their guns. Use what works for you.
Many people find themselves wondering what to use as an actual starting charge.
If that isn't possible, one option is to begin with a load data for shot loads. In a 20 gauge, the standard load calls for a 1 ounce charge of shot. This will give you a comfortable safety margin. Most folks also use a 1 ounce shot charge in the 12 bore. In the smaller bores, such as the 24 gauge trade gun or maybe a 28 gauge smooth rifle, start with the same charges you would start with when working up loads for a rifle of that caliber.