You probably can't print a "perfect" 90 degree overhang simply because of the mechanics of the Peachy printer - the water level is rising continuously (the printer has no control over that) and it's going to take time to print a large, flat plane. As a result, whichever bits get printed last will be higher. For accuracy, I suspect the goal will be to print a lot of layers, with minimal time spent on each one - so that the height change during each layer is kept as small as possible.