I'm not sure I understand what you mean by having to keep your bow arm straight in order to maintain anchor. Anchor is a function of your drawing hand/arm/back muscles. Your bow arm shouldn't have anything to do with it. Your anchor point can be just about anywhere that is solid enough to be repeatable from shot to shot. As Ray said, we are all different and have to modify our form to find what works for us. The key is doing the same thing each and every time.
That said, there are certain principles or basics that come in handy. The bow hand, drawing hand, and drawing arm elbow should lie in a straight line with the arrow. In other words, the pull should be straight back. The draw starts with the arm muscles and then the load transfers to the back muscles as the draw is completed. This line should be parallel to a line drawn through the shoulders. That is what Terry's clock demonstrates.
As a side note, I was watching some archery videos on YouTube the other day, many of them showing archers from Asia. At first I thought that most of them violated everything I had believed about shooting a bow. The drawing hand would fly back violently and the arm would sometimes end up extended almost straight back. In some cases the bow arm would also appear to fly to the side and the bow would rotate in the hand until the string was out sideways or beyond. Anchor points seemed to be anywhere from corner of the mouth to the ear to one that appeared to float between the head and collar bone. There was, however, one factor that was the same in each case. They all showed the same alignment of the bow hand/arrow/drawing hand/drawing arm elbow that shows in Terry's form clock. This tells me that as long as your basic form is good you can do a lot of other things "wrong" and still hit what you are shooting at.
John