It is many years since I have done any gliding and, in my day, there were two distinct factions who spent many bar hours arguing about the correct way to carry out the approach to land.
One side supported the square circuit, carrying out S turns at the end of the downwind leg if necessary.
The other argued for S turns on base leg as DFC appears to be advocating.
As an ATC gliding instructor I was, and still am as far as PFL's are concerned, very much in the first camp for the following reason:
180 degree turns are not the best manoeuvres, especially under near panic conditions, for controlling speed, position and rate of descent , making it easy to misjudge the approach into the field. . It would not surprise me if most of the spins off final approach aren't due to having to turn too late and too low. However, if you have completely misjudged and are far too high, then this one way to recover
Before the 1000' point on downwind, level with the touch down point, it is easier to assess the conditions and then adjust final turn. As FD says, by 500' you should really be "in the slot".
In my last biennual flight with an instructor I spent almost the whole flight carrying out PFL's from a range of heights. For one we deliberately started the final turn much too high and resorted to S turns. Yes it worked but I would much rather make those sort of adjustments above 1000' where it's safer.