following my blog here;
http://socpuppet.blogspot.com/2013/09/juniper-srx-vpn-automation-tool.html
and
http://socpuppet.blogspot.com/2013/09/vpn-ikev2-juniper-to-fortigate-routevpn.html
http://socpuppet.blogspot.com/2013/09/vpn-ikev2-juniper-to-fortigate-routevpn_10.html
keep in mind the above is based on a
routed-vpn-mode and using ikev2 w/PSK, if you want ikev1, minor changes. If you want certificates based authentication you will need to defined certificates and apply these both for the SRX and FGT
I will work on a post later on this month with certificate based authentication and either with pfsense or openswan, since those are the only two that I;' ve done certificates lately.
But the above links will get you started.