Please look in the "box" post. Theory is a wonderful thing in terms of logic. But it can be a real pain in the butt when it comes to playing. All music is sound, listening!!
It is quicker to just do. Any reasonable ear can discern what is being heard. In time you should be able to sing.
Do NOT be concerned as to where you should start a scale.
In time the ear will determine your choices. I have a severe habit of playing these scales from the 3rd, 7th or extension. The key is not what you play, but how you resolve. You start at one point, reach the pentultimate and end. The ending is what makes what you play make sense. This will always be a split second decision. In time, you'll
get good at it.