As Rolf mentioned 5.4 would be OK but you could probably go as low as 4.2 in a yarn tempered rip stop dacron.
The full length battens do a lot to protect the cloth in the jib as they tend to stop the jib from flogging.
As far as cutting dacron just like cutting glass fabric I have found a rotary cutter very quick and handy. It is also really good at cutting uni directional carbon without disturbing the fibres.
Uni Carbon really doesn't have anything to do with making sails but it is handy to know as when working with Uni carbon it does not like to be handled any more that neccessary.
Good luck with the jib.

Regards,
Phill

I know that the voices in my head aint real,
but they have some pretty good ideas.
There is no such thing as a quick fix and I've never had free lunch!