What you describe sounds to be as the circuit is generally taught.
A few possibilities:
Is there a wind component down the base leg? If so, try to recognise it early and widen your downwind leg accordingly.
Is your downwind to base turn a smart 30 degree banked turn?
A 20 degree bank will give you a much bigger turn radius and put you closer to final at the end of the turn.
Taking some flap at the end of the downwind leg will help to keep your circuit tight, but this is usually taught at a little later in the course and slowing too much in the circuit could cause bunching if the circuit is crowded.
It should all come with practice. If your positioning in the circuit is good then adjusting your power settings and flap selection for the conditions of the day should be sufficient to keep your approach profile stable and consistent. 1500 RPM is just a starting point, but won't be correct for every case.