So i bought 2 of the selkirk 6"wall thimbles..it says to frame a hole 12x1/8 x 12x1/8...is this necc? Whats the best way to approach this? Also someone mentioned 2" gap somewhere??? These are class a selkirk thimbles part # 206463 im new to this and need some help...i marked out 18" from corner wall and ceiling...and found my right side stud to start install....havent done any sheetrock cutting yet