In my opinion the 225lb reference is incorrect. "The Good Mornings are a good excercise for the right person," that is what Jon Kuc said in his book "Kuc Speaks." It is known as a good morning because of the movement in the erector spinae which resembles the rise out of bed to stretch. The hips and head extend beyond the rectangle, placing more stress on the back, because of it’s exaggerated horizontal angle. Please join this discussion about good morning weight VS squat weight within the Weight Training & Weight Lifting category. The good morning is one of the most maximally effective lifts you can do for total muscular development, and now you have no excuse not to do it. The good morning is a versatile hip hinging movement with a multitude of benefits, not a half-assed low back squat without knee bend. If squat volume is already high then you can remove some of your good-morning squat volume and replace it with front squat volume - if not then just add front squats on top of what is already there. A squat pattern will place more emphasis on the knee joint: A hip hinge pattern will start at the hip and place more emphasis there, hence the naming of the pattern itself: While there's obviously movement happening at both the hips and knees, the primary joint leading the movement itself is the distinguishing factor when grouping the patterns. But if your goals involve getting stronger or attaining a more aesthetically pleasing backside, then the good morning is a lift you need to be doing. Personally I'm in the "I can move more weight if I avoid good morning in a 1RM" camp but was having trouble avoiding it in my volume work. The athletes best Clean max should normally be about 80% of their best squat max to be in a good lower body structural balance.

The good-morning back squat The barbell good morning is a serious posterior chain movement that is too often neglected by the strength athlete.
Simmons recommends frequent, heavy use of the good morning on max effort days — where the lifter works up to a 1-5 rep max — to help improve the squat and deadlift. The squat becomes a hybrid between a good morning and a back squat, as demonstrated by the stick figures in the highlighted red boxes in the diagram below.
The good-morning is a weight training exercise. Excerpt: what sort of difference in weight is there between a competion or 1RM squat Vs a 1RM of a standard good morning(on max effort day)? Good Mornings were trained at up to about 60% of the athletes best Clean max.

The erector spinae muscles of the lower back work isometrically to keep the spine in an extended position while the hamstrings and gluteus maximus work isotonically to perform hip extension. So, if you break down the percentages, a good training weight on Good Mornings should be at about 45%-50% of your squat max. A Shw that squats 900 and a 165'er who squats 400 are two very different examples.