With the elko i mean the electrolytic capacitor of the output filter .
Since you have to do with a square wave the amplitude is decided when you calculate the transformer .
With that i mean that except for line variations the peakvalue(voltage) is decided by the volt's / winding .
(as example) even if there is lets say 4,2 volt/winding
Then that will just mean that for normal functioning the duty cycle wont be above 93 % .
4,3 volt * 3 windings = 12,9 * max duty cycle .
But that also means that since the designer knows the absolute maximum voltage to be expected the capacitor doesnt need that much overhead voltage .
After all worst to expect is Npri/Nsec*Duty cycle +- line variation
For a 12 volt powersupply to make it jump to 16 volt (assuming 240 Volt supply) it needs to jump to 300 Volt AC .
And even then it will be just a few pulses till the feedback kicks in and lowers the duty cycle till the voltage drops .
So probably your output condensator is either 16 or 25 Volt (if you didn't change it anyway)
It might be the source of your hissing (then again i didn,t see your project so its a bit of a wild guess)
Maybe its just a stupid thought of me (for myself i could forget things like that and just fire the thing up after i changed it)
For the calculations of the toroidal filter (i guess you mean the output filter)
Open Pressman's switching powersupply design 2nd edition and on page 77 of the file you have the proper formula .
Also read from page 103 onwards the chapter about halfbridge topology
(sorry for the paperwork but i can asure you that your understanding of the circuit at hand will improve by doing so)
Good luck and above all be carefull .