If you were doing an extract kit partial boil with top off, you didn't need to add extra water. If you used all the extract in the kit, you WILL end up in the correct OG range if you end up with the proper wort volume. There is no OG to "hit" in the circumstance. Just use all the indgedients and up up with the correct volume. All you did was thin out your beer.
It is very hard to get top off water and the boiled wort to mix perfectly, leading to strange gravity readings - usually on the high side.
There are no effeciency issues to worry about in extract brewing, and even if you got more steeping grains that you should have, it wouldn't make a big difference.
In other words, as long as the kit OG was figured correctly, there simply is no way to end up with the wrong OG as long as you used all the ingredients, and ended up with the proper volume.
So.....when doing an extract with grains partial boil with top off, there is no reason at all to even take an OG. Really. The kit ALREADY has it figured for you.
Pez.