I'd keep the offset at +0 or higher. You start getting more aggressive than that and rubbing may be an issue. These 14s have tight spaces is only downfall on this design.
This is kinda of a blanket comment that is totally inaccurate. The new lifts are using drop ca brackets which is great idea. All lifted trucks will not be as smooth as stock unless your dropping big money on good shocks and struts for fronts or coilovers for that matter. I have a buddy running a rc lift, not even sure the year, and has gotta have well over 100k on it and not any issues.
No prob. Easy to figure out bs in future. Whatever your advertised wheel width says always add +1. Remember 1 inch converts to 25.4 mm
So yours would be -12/25.4=-.47. Now take your wheel 9+1=10/2=5. So 5-.47=4.53 bs. You can take off your wheels right now. Get straight edge and measure from that to mounting pad inside wheel. Easy to do and leaves no room for error. Your offset of -12 basically means your mounting pad sits inside .47 of centerline of the wheel