In order to answer this question you need to have access to a t critical value table that gives t values for varying confidence intervals and degrees of freedom. I have included one in the reference links for you.
When your desired value lays between two points in the table the common practice is to take the higher value of the two as this gives you the least optimistic test of significance.
For a confidence interval of 92% with 15 degrees of freedom the critcial value of t is 1.75 if you take the higher value of the two (as 92% (0.08) lies between 90% (0.1) and 95% (0.05)).
If you require a more accurate response to your question a little bit of mathematics can get you the exact t value:
Rearranging for x we get:
`x = 1.34 - (0.1-0.08)(1.34-1.75)/(0.1-0.05)`
Therefore, for a 92% confidence interval the critical value of t is 1.50