On a lot of airfields, use of landmarks is inevitable. This is not for flight safety or training reasons, but to ensure that the local sensitive noise areas are avoided. Old Sarum is a good example of this, which has a necessarily rather non-rectangular circuit, at least to the South.
Mostly however, a good rule of thumb is to turn crosswind when high enough to turn safely, and turn base when the threshold is 45° behind you. Downwind leg distance should ideally be as close to the runway as you feel comfortable, but mostly will simply be in line with the chap in front of you.
Also, it is never a bad thing to extend on climbout, or downwind (or exceptionally crosswind) if the circuit is crowded or you are faster than the other traffic, so as to give you safe separation from the traffic in front.
G