Hi thats a form of the general formula:
total_lift_required = weight_of_payload + weight_of_balloon + a_bit
neck_lift_required = weight_of_payload + a_bit
where "a_bit" is some amount. I've seen both a fixed amount or some proportion of the payload weight used (50% payload weight here). At best its a gross simplification that probably works OK for midrange flights - but it takes no account of the desired ascent rate or lifting gas used. For example using it for a very light payload on a large balloon will give virtually no ascent rate.
You might want to vary ascent rate depending on type of payload (photographic or not) - or to reduce or extend flight drift. A mid range value for ascent rate is about 5m/sec = 1000ft/min approx.
A much closer estimate is to use the calculator at the bottom of this page:
http://randomaerospace.com/Random_Aerospace/Balloons.html - this gives results in cu m and cu ft.
http://habhub.org/calc/ - which just gives results in cu m (note 1cu m = 35.3 cu ft)
This calculator works by estimating the size of the inflated balloon and approximating its average drag forces through the flight. It combines this with the lifting force of the balloon and weight of the payload to calculate the amount of gas needed and what the neck lift will be. It also calculates an estimate of balloon burst height.
In reality several other factors (expansion acceleration, drag crisis, skin tension, thermal flow ...) affect the ascent rate - so even though the calculator gives a much better estimate its not 100% accurate.