The initial baseline income, for the delineated area within which the retention is to take place, will be set through an initial baseline income return.
Bydd yr incwm sylfaenol cychwynnol, ar gyfer yr ardal a amlinellir y bydd ardrethi'n cael eu cadw ynddi, yn cael ei bennu drwy ffurflen incwm sylfaenol cychwynnol.