Right, where to start?
Firstly, we can find details of how to calculate cures here:
http://www.fsis.usda.gov/OPPDE/rdad/FSI ... 7620-3.pdf
The info we want starts at the bottom of page 21 (of the document - the page number may appear differently in Adobe reader). We see here that there are two ways to calculate immersion cures. Much debate, messing about, and a bit of serious testing leads us to believe that 'method 2' is appropriate for your meat (even though some of the info may contradict this).
The formula we apply is therefore:
Using this will give the amount of nitrite in Parts Per Million (mg/kg) if the meat is left until it reaches equilibrium - a time we believe to be well in excess of 12 days per kg.
You say that you used 5% salt, 2.5% sugar and .5% cure per 1ltr water. For ease I calculated your 3 ltr brine as 3ltr water, 150gm salt, 75gm sugar and 15gm cure - 3240gm in total. You say your meat was 1780gm. We understand the cure #1 to be 1:16 nitrite: salt (5.88%).
So our calculation is:
(15 x 5.88%) * 1,000,000 / (1780 + 3240)
That's
0.882 * 1,000,000 = 822000 / 5020 = 163.74 PPM (mg/kg)
I said 175 PPM before - that's what comes of trying to work it out in your head!
The US rules tell us that when this happens the nitrite/salt/sugar etc in the meat will equal that in the water. That is, to gain any more the meat will actually be sucking the nitrite/salt/sugar out of the water against the natural flow (so to speak!).
As such, I believe that with this method of curing the maximum is unlikely to be more that this.
Yannis is of course correct that the "absolute maximum" is around 500PPM. You would know if that had happened though as the 'brine' you were left with would just be water!
HTH
Phil