I'm in a similar position of FIREing at 52 with DB and state pensions coming along later. For me it works much better to think in terms of two separate pots - a 'perpetuity' pot, handled to give an income at your desired withdrawal rate over your whole retirement, and a bridging pot specifically designed to be spent down over the gap.
As an example, if your desired income was £20k pa, half that was covered by DB/SP and you had a 15 year gap to fill and used a 3.33% withdrawal rate then you'd be looking for £300k in pot 1 to give you £10k drawdown, plus £150k in pot 2 to give you the other £10k for the first 15 years. My personal slant is to invest pot 2 much more cautiously, but that's just me.