The General Council on Finance and Administration (GCFA) recommends "the formulas by which all apportionments to the annual conferences shall be determined, subject to the approval of the General Conference." (¶ 806.1c, 2016 Book of Discipline)
The apportionment formula used by GCFA is in the Financial Commitment of The United Methodist Church 2017-2020 booklet.
Each annual conference has the authority to decide its own formula for the apportionments to local churches within its boundary. Annual conferences can use different approaches to determining the formula; for example, some use membership or church financial health. The annual conference formula is in each conference journal. If you have questions about your annual conference apportionments, talk with your conference treasurer.