I've started to simply use my hydrometer when topping up (correcting for temperature difference from the hydrometer's 15C basis, with my glass thermometer handy). This way, as long as I've dissolved the kit and stirred well, I know I'm brewing as close to the intended OG of the kit as possible. Any slight variation between individual batches in the manufacturing is even taken care of this way, small though that may be. Far better than topping up with an arbitrary amount of water, in my opinion - Just seems like shooting blind-folded to me.
I have just bought my second Wherry kit, plus another sachet of Gervin ale yeast (to deal with the 'under-pitch' of the 6g supplied in the kit) as before. Although still conditioning, my first one is really joyous and tasted good from the moment it was down to 1.012. I dry-hopped it with 20g Goldings pellets for a few days in a secondary FV which has added a lovely aroma, but it was looking very good even before that. I suspect this will be well up there with my best extract brews in a few weeks time.
That said, I still agree with what was said above: I don't think an extra litre would have totally spoiled your brew.