How Do I Know Enough if Excess Skin Has Been Removed from my Tummy Tuck?

There are many reasons for this. TYpically loss of elasticity that is seen with age, sun exposure, massive weight gain/loss, pregnancy may be more noticeable when you bend over or sit down and will not always be eradicated with a tummy tuck alone
 

First of all, it is normal to bend over and have some skin excess, even after tummy tuck surgery. Otherwise you wouldn't be able to stand up straight without some serious tension on your incision. As far as a vertical limb closure, if things can't be closed easily without tension at the time of surgery, this approach may be required. This can be predicted ahead of time in susceptible individuals with only mild skin excess, therefore there are no surprises. I agree, that a low position of the abdominal scar is desired. If I end up with a vertical limb at the time of surgery (which is rare), it is usually at the bottom midline, and can be removed and revised later, after the skin has healed well and is relaxed to allow a primary tension-free closure.

Tummy Tucks do not correct problems with loss of elasticity of the abdominal skin. The goal of the operation is to remove excess inelastic skin and to tighten the abdominal wall musculature, which creates a flatter contoured abdomen. The skin roll that you are describing is most likely related to this loss of elasticity Over stretching the skin can contribute to an increase in this condition not enough can result in an unsatisfactory result.


In my experience, post operative abdominal compression may improve the contour of this inelastic skin to some extent, however some visual laxity is certainly expected when bending 90 degrees in such cases.

What concerns me about your posting is that an experienced plastic surgeon who has done lots of tummy tucks should almost never be surprised by what they find during surgery and require modifications they hadn't predicted for you before surgery.  This kind of intraoperative change of plan should very rarely happen in experienced hands.