First, I have not seen anyone 'encourage' the idea of topping off your beer going into secondary. It totally throws off the calculation for ABV as well as the color, flavor, aroma, and bitterness, unless you have counted on that in your initial brewing. I do know someone who deliberately brews their beer at 2x strength they want it to be in the end and then dilute it when putting it into the keg, but it is not something I would recommend unless you have experience doing it.
Since you don't have a final gravity of the beer before diluting it, you won't be able to just do a straight dilution calculation.
Getting back to your question, the way to "calculate" your approximate ABV is to open up your recipe in BeerSmith, plug in the amount of water you added into the "top up water" in the fermenting/bottling section of the 'volumes' tab. Now go back and change the amount into the fermenter on the design page to be the original amount PLUS the volume you just added to the "top up water". This should give you a fairly good idea of the resultant ABV.