A slight sag between rafters is ideal as it will allow any water to drain down the felt.
How to felt and batten a hip roof.
When the first length is complete lay the next layer of felt on top of the first so that it overlaps by at least 100mm 4 inches horizontal overlaps where one roll ends and the next one is started should be at least 150mm 6 inches.
There will be additional excess at the top of the strip since you re folding it over a sloped hip ridge.
Work along the roof taking out any excess slack in the felt but do not pull it tight.