It is also worth noting that the point where a descent from MDA should be initiated will be affected by the height above the rwy, the groundspeed and the required descent gradient. A 500' HAA MDA would normally call for descent to begin no further than about 1 1/2 track miles from touchdown to achieve a 3 degree descent at 140 or so knots. this would normally occur with less than 90 degrees of turn remaining to line up on final. With an MDA of 1000' HAA, the descent would have to begin about 3 track miles from touchdown. In order to remain within the protected airspace and maintain at least a 3 degree descent angle, descent initiation would need to occur somewhere on the downwind portion of the maneuver. Some mountain airports have signifigantly higher MDAs which require even more track miles to descend at anything approaching "normal" rates.
Best regards,
Westhawk