When to turn from crosswind to downwind to base in a circuit?
Like almost everything (aviation or otherwise), this is just something you learn with practice!

I was taught to turn downwind to base when the threshold was between 4 &amp; 5 o'clock (in a RH circuit). 

Here's a good graphical representation in a LH circuit. 

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  At the end of your downwind leg, when you once again reach a 45° angle to the runway, do a medium 30° turn onto base.


(Image &amp; Text source: http://www.ppl-flight-training.com/circuits-briefing.html)

As for the base leg being high or low - it shouldn't be a problem. You can easily correct this. 

Too high:reduce power 50-100rpm, lower the nose slightly to maintain speed
Too low: Increase the power slightly.

As was commented, it is also worth considering the wind direction before you depart. On a perfect day, the wind will be directly down the runway towards you on final. Consider the effect of this on your circuit? I find it useful to draw the circuit, and an arrow with the wind direction. Then consider when to turn early/late as appropriate.

Finally remember the GOLDEN rule: You can ALWAYS go around. (Assuming you have a fan up front!)