I'm personally a proponent of 'RPing stuff out' with as little interference from stats and whatnot - but I think a tier-system may help since each player seems to already have developed their own. This would just be a matter of unifying everything and making it more streamlined. It would simply be a reference of base 'skill' rather than something that would decide battles and duels in future.

For instance: Aulus, being young and inexperienced but well-trained, may be a Tier 1 in the -10 to 10 system
A veteran soldier, being experienced and trained, may be a Tier 2

But Aulus killed two of the above in the last post using trickery xP it would simply act as a measure of 'pure' skill stripped of context. I believe the Tier 1-5 in the reference sheet is likewise stripped of context.