Index has been trending down recently and today I met a personal goal and milestone: posted my first negative differential score. It's a combined score of the front 9 at one course where I was 1 under, and the back 9 of my home course this morning, where I was 1 over, for a -1.2 diff 72 (73.4/130).
I am of the opinion that combined 9s are dumb, at least for me, because when I play 18 I tend to taper off and suck on the last 3-4 holes, which can't happen in a quick 9. Still, it counts under the rules. It looks cool!