I agree with rshdunlop, don't bother factoring in the 356 bus as part of your commute. Have you checked the distance to Sydenham station? From the junction of Perry Rise and Perry Hill it looks like it's a mile to both FH and Syd stations. Sydenham is also in Zone 3; you'd avoid the steep walk up Perry Rise in the mornings; and there are more buses between Bell Green and Sydenham station. I don't know what traffic is like in the mornings but I doubt it's worse than the South Circular going into Catford.

Catford is slightly further (1.2 miles) but that gives you a choice of Catford and Catford Bridge stations. Wouldn't want to rely on getting there by bus in the morning though.