Glad to see this thread is still alive and well, and most of all helping some people!!!
Ok, I feel rather rusty, BUT, in exam situations (when you are short of time) I'd take the 68 as being half way between the two values (70t @ +10 25500 and 65t @ +10 27800) and just weigh it to the heavier as it favors the heavier side. So a basic (25500 + 27800) / 2 = 26650 and then just favor the lower altitude because you are heavier, which in this case is 26400.
Or if you want to do it correctly just apply do
((25500 - 27800) / 5) * 3 + 27800
Which is 26420.
Either way works though the shortcut can put you in no-mans-land in an exam, so beware.
Also, make sure you read the question, it could quite well ask for the lowest allowable flight level, and given that you were flying 310, 260 would not be legal.
Haha, noticed Swift beat me to it posting the full interpolation method
