There will be additional excess at the top of the strip since you re folding it over a sloped hip ridge.
How to felt and batten a hip roof.
Work along the roof taking out any excess slack in the felt but do not pull it tight.
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.
A slight sag between rafters is ideal as it will allow any water to drain down the felt.