Does the boiling boint of miscible liquids follow a linear relationship?
Except if molecular composition of your two liquids are similar, the boiling point of the mixture will not proceeds linearly but like this for instance : 



With the lower blue curve the boiling curve. In between the two curve you'll then have a mixture of vapor and liquid and only vapor on the upper part the binary diagram. 

So you should find out an already drawn binary diagram for your mixture or draw it out by yourself first.