Sparge Rate

I have been all grain brewing for a couple months now, and have been doing a batch sparge so far. This weekend I built a, for lack of a better term, Sparge Arm so that I can start doing a fly sparge vs batch. It doesn't rotate or anything, but I get pretty good coverage of the whole grain bed (well where the grain bed will be).

A couple questions:

1. I am brewing 6 gallon batches, and my total brew kettle volume is around 7.5 gallons to achieve this. I typically mash with 4 gallons, and sparge with 4.25 gallons. It gets me pretty close to the 7.5 at the end. Is this right, or should I go a little more on the mash and a little less on the sparge?

2. When do I start sparging? I recirculate 3-4 quarts to set the bed, then do a slow drain in to my kettle. Once I start the slow draining of my mash tun, should I start the sparge at the same time, or wait until I can see the grain bed, or when there is still a little water over top of the bed?

3. At what rate should I sparge? Is 4.25 gallons over 30 minutes too fast? Should I shoot for 45 minutes or an hour for that volume?

Thanks in advance for everyone's thoughts.

1. Whatever works. Looks like you'll be doing a hybrid system where you mash in as if it was a batch sparge, but wash the grain as if it was a fly. There's nothing wrong with that.  As long as you hit the right temperatures I don't see why the method is that crucial. 

2. The rule of thumb from the Papazian books is to keep an inch or two of clear water over the grain.  So I'd say drain until you're a couple inches above the grain, then start the fly.

3. I start my boil with about the same kettle volume, and I aim for 45 to an hour total drainage time.  I'd rather go slower than I have to and be on the safe side than run it too quick and have a stuck sparge.

Fly sparge involves a balancing act between drain rate and the adding of fresh water.  So it's not fire and forget. You got to babysit it.  Above I said the general rule is a couple inches of water, but it's not that crucial as long as you're not going to overflow and make a mess.  You definitely don't want the drain rate to exceed the fresh water. 

Looks to me like you know what you're doing. I don't anticipate you having any problems.

Thanks for the reply Maine Homebrewer!