If Table D does not have the degrees of freedom (df) you require, use a df as close as you can get without going over.  Which is to say, use a smaller df that is on the table.

 

For example, if your n = 100, telling you that df = 99, but there is no df = 99 on Table D, you have to make do with df = 80, the closest you can get to 99 without going over.  Do not use df = 100 even though that is closer.  Never use a df that is higher than what you actually possess.