Do this:
Add 3.5kg to 12L of water (at your calculated strike temp) and get rid of any doughballs. Write everything down. Strike temp, mash temp, water volume, grain temp, any weird things etc etc.
At the end of your mash, squeeze the crap out of the bag and retun it all to the pot. Pour 2L of hot tap water through the bag, squeeze again and return this to the pot. Pot should have 10-11L of wort in it.
Add hot tap water, or boiling coffee kettle water until it's at 14L. Measure your SG at this point. Then you will know the final volumje fo your batch. Boil and add hops.
The reason for all this guesswork and writing is that ... you (and I) don't know what your efficiency will be. You might get 14L of 1.055; you might get 14L of 1.045, and this will affect your final dilution in the fermenter.
High gravity Stovetop brewing is about tweaking the volume in the fermenter.
Remember, you need to calculate your Strike Temp (mash water), the IBUs (bitterness) you will expect from the hop additions, correct any SG readings for temperature, and your dilution rates in the fermenter. Learning all this will give you a better appreciation for brewing than the folks who use beersmith to do the thinking for them. This helps with future recipe formulation and advanced mashing techniques.
Get stuck in! Don't worry about it too much - you'll make beer - but write everything down! This will let you isolate problems and fix them next time. If you just brew-by-numbers you will take longer to understand some of the finer points of AG. These finer points are the difference between good and great beer.
I don't recommend using beersmith initially as it abstracts your learning process by feeding you numbers you didn't calculate manually (that relate to your gear and your techniques).