Hildy,
as has been said above- id ditch the gps...at least until you have finished your flight, then you can review it, but i wouldnt use it in flight.
The constant viewing of the runway is what will determine your circuit performance. The gps is going to add too much to your mind at this stage, and can be a hinderance.
With regard to how long before turning, etc in the circuit, this is the approach i use.....continue on upwind until 500ft has been reached.....turn onto crosswind, looking underneath my wing, ( i fly a Jab), until i can see the runway appear....level the wings...this concept usually gives a pretty good 90 degree line from the upwind point.
when i have reached 45 degrees from the end of the runway, is when ill turn downwind....roll the wings level, to track parallel to the runway....always looking at that runway throughout downwind to determine any drift you may encounter.......at 45 degrees from your threshold...turn onto base, again giving you a nice 90 degree line to the runway. the turn onto final is pretty simple, and you will pick up the turning points eventually, as the wind will always play games with you. Usually, in the Jab with an approach speed of 65-70knts, i aim to turn final at 650ft, which should allow for an established, wings level line on final, by 500ft.
i know its a bit long, but i hope it helps.
Liz.