Figure 1 for the vent slot cut now you are down to 2 to lap onto the shingles and keep the weather out.
How to install mobile home roof vent.
Give up 2 on each side for the 2x8 etc you are left with 3.
Take apart the roof use.
Rent a dumpster for this project you need to rent a dumpster in order to toss the old roofing materials into.
Climb on the roof you will need a ladder that is tall enough to allow you to easily climb on your roof to remove.
With your widest ridgevent material at 10 allow for 5 on each side.
Rent a dumpster step 1.
So your cut would need to be down below that on each side.