If you read the blurb on felt roofs, it has been recommended that the primary layer is fixed with small clouts.
This is to counter the movement differences of timber(ply) decking and the felt which are totally different.If memory serves, every 12", or 15"both ways.
To last a really long time, I use Wickes felt, I would lay the first course from the gutter, up to the ridge, back down to the gutter line, and overlap on the 100mm chalked line, use enough nails to secure only.
This will allow the second course to be in sync with you're shingles, laid across ways, and securely fixed at, say, every 12". Make up and fix the perimeter welts, folded, and secret nailed to the treated 2" x 1"along the fascia.
In you're situation I would not use lead soakers, Clean and prime the marked strip you'll be fixing to and use alcan foil
flashing, Good stuff when people use the primer, I have found, and easy to repair, a few years down the line.
Completely stick, and seal the ends using Wickes gutter sealant, which sticks like the proverbial, but is very flexible.
I've probably bogged you down, I guess a Google search will bring this all up for you.
HTH Regards Rodders