Being newly shingled, I would strive to easily pry up, with out tearing, the main house shingles in order to get at least a 6 inch overlap of the main house shingle over the new roll shingle of the porch roof. I would even strive to go up about three courses on the main house and there begin your underlap so that the rush of the water will carry on from the main to the porch and rush off. This will prevent any leak at that crucial intersection of the main to the flat porch. I do not like the flashing idea because you need to get the flashing up at least four inches to go up and beyond the throats of the comp main house shingles. You may have to even break some of the nails to achiever this.
Then on the end or fall off of the flat porch roof, you need to install the drip edge flashing all around the edge and fasten it with those roofing nails with rubber groments. Then bring your roll flashing down over this drip edge and extend over about 1/2 or more. This will ensure that the drips fall from the leading edge and do not walk back and end up rotting the wood.
Flat roofs are nortorious for leaking IF not done correctly and overlap is crucial. I think that the procedures for overlap on roll shinges is about one foot or more.