Seems like the three examples you gave are all pretty similar.
If you have 2 of any objective set in your deck, and haven't buried one then the probability of flipping it next is:
2/6 = 1/3 or 33% if only 10 objectives
2/7 = 28.6% if you have 11
2/8 = 1/4 or 25 % if you have 12
If you buried 1 of the 2, then the probability of flipping it next is:
1/6 = 16.7% if only 10 objectives
1/7 = 14.3% if you have 11
1/8 = 12.5% if you have 12.
For all those cases every time you flip an objective and it's not the one you're looking for, to find the new probability, decrease the denominator by 1. So if you started with 10, didn't bury one of the 2 and the next one you flip isn't 1 of the two either, the probability of flipping it the next time is 2/5 or 40%, etc..