Well, officially it's: 10+(Creatures HD/2)+(Creatures Con modifier). AS it is written in the MM on page 9. Now the precise effect of the poison is what makes it deadly.
Such as
Primary - - - - - Secodary
2d6 Con - - - - - 2d6 Con
1d10 Con - - - - Death
And so on...
Besides you can always give the creature an extraordinary ability like the following:
Potent Venom (Ex): The creatures poison attacks are more potent than normal, increasing the base DC with (Insert number here.)
Or, perhaps that would be better suited as a feat?
Hope I was of assistance.
Later,