Tom,
I always have converted 30 inches to mm. Never thought about it your way, no reason you could not use the 768. Yes, you still get 3 pieces per sheet.
Jon,
I believe the reason for the change in height is to finish out the 3 cm tops at 36". As for the depth, if you have a 24' deep box and add the door, it will finish out about 24-3/4". The biggest change I have seen over the last few years is in vanity height.
Rocky,
I neglected to mention that you should trim one long edge of the new sheet, then square a short edge.
Tom