Like the others, I did this for my first AG batches. But there it was even anticipated since they were higher gravity beers (Doppelbock and Maibock).
When I grind the grain at the LHBS I inspect the grind and make shure it is ok. So far my efficiency was pretty consistent. And since I batch sparge, there is not much that can go wrong. So I stopped keeping DME at hand. After I collected 24L (~6.5 gal) of wort, I measure its gravity. Then I determine what the post boil volume should be and start the timed boil. If I don't expect to boil down to the necessary volume, I would have to extend the boil. At 20-15 min left, before I place the chiller into the boil, I check the volume in the kettle and start boiling the amount of water I need to add at the end to get to the anticipated post boil volume and gravity. I then add the water when it is boiling. Usually I need to add 2-3L (2-3 qts).
For starters, I usually have wort left over or may run the lauter for a final time.