Buddy you did good. Flats are the most persnickety things to get right - on moisture first and texture second. The protective and self basting barrier of the point meat is a must. I bet if you just cut off only what you needed it would have been more moist. In addition, rubs and marinades have a drying effect as well oddly enough. People expect more and they can add moisture to grilled meats but in a long cook its not a great thing to do. This also can happen if your rub is a high salt rub. This is one reason why injections help some bbqers more than marinades.
Some still think that marinades don't even matter on these large cuts.
Popdaddy is Dead - 1933-2011 - Pitmaster T is a free agent