The fastest angle is broader than most people think, at least on flatish water with plenty of wind. It is more than 120 degrees and possibly up to 135.
One of the best ways to judge the best angle is to physically watch your GPS as you bear away. Often it feels like you are slowing but the speed readout keeps increasing.
It is often possible to get a rough idea of your sailing angle using GPSAR. If the wind angle is close to correct you can turn on the sailing angle and see where your best runs were. The problem with this approach is that often the actual wind angle varies through the day and even between runs, so it is a guide only.

I think it depends on how strong the wind is, when it's light you can't bear away as much as when it's really windy. Having a big flat expanse like Hardies Run helps, you can pick the best angle for that particular wind strength, without having a sand bank to limit your choices.

I've gone too deep on a light day, and found myself slowing down, bear back upwind and bang up goes your speed. When a gust hits you can go deeper as well.

If you want to watch your GPS then it has to be on your forward arm (duh!) I like to have it on my back arm so that on the run back upwind you can look at it and figure out what your top speed was, click the toggle to save the number and then reset the max speed, without having to stop planing... one alpha attempt later and you're back on the downwind speed run