I've never used FW's 555 but I have made very good prime15 clones with hangsen No5 and highway and adding a little AP and inawera chocolate to it. All I can suggest is maybe start at 5% and adjust up or down from there.
you'll find some pretty good halo clone recipes including a few of my own in this thread
Halo Freedom/Smooth/Torque recipes???
Here's a few examples
7% hangsen highway
3% Hangsen cocoa
1% Methylbutyraldehyde (get it here)
Perfumers Apprentice - Methylbutyraldehyde 1% (PG)
2% AP (Get it here) Perfumers Apprentice - Acetyl Pyrazine 5% (PG)
75pg/25vg/18mg base
7% tobacco absolute (If you use TFA's thick tar TA dilute 2 large drops in 10ml PG and use that as your concentrate)
4% AP
4% hangsen cocoa
2% MTB (Methylbutyraldehyde 1%)
4.5 - 5% Hangsen T5
1% AP
1% TFS - Sweetner
The discussions start at about page 19.