You need to work backwards from when you have to be Stable ie on-slope (3°) at 500(?)ft. That will define where you roll out of the base turn, then work back to the base turn start point, then the groundspeed on Downwind will define how long you go Downwind for. As mentioned previously, this may mean you fly a level Base turn initially, then start the final descent when intercepting the 3° path, hopefully to roll out at 500ft (or whatever your Stable rule is), on slope. For example, if circling at 1000ft, start down half way round the turn. If circling at 600ft, start as you approach Final.