This is an interesting question, as I'm never sure what the best answer is. I tend to vary my retrieve from cast to cast (something like Lgraydonflyfishing, but less consistently). Sometimes I start retrieving immediately, sometimes halfway down the drift of the fly, and sometimes I let the flies swing right round and only start retrieving when they are almost straight downstream of me. All methods seem to work sometimes, but in almost all cases the likeliest time for a take is right at the start of the retrieve. I suspect that if we could see the fish we are targeting, we would aim to get the fly to swing into its vision and then give it a jerk to prompt the take.
I usually start F8 as soon as the flies touch water to keep in contact with flies.
Most of my takes are usually in the first 5 or 6ft of the swing, occasionally I get a take in the last part of swing.
Likewise what others have already said, depends on the height & speed of the current to what casting angle i fish.