Hi Jack,
When you talk about things being "in line" or "aligned," you need to reference two different lines, not just two points. If I am standing on my right foot, wherever I put my left foot down, whether to the right, left, front, back, near, or far, I will always be able to draw a line between the heel of the front foot and the back heel or toes of the other foot. There are an infinite number of such lines. From what you describe, one of these lines is somehow special to you, but I do not know which one and cannot tell from the picture since there is nothing particularly distinctive about the two lines you have drawn.
If you are standing on your right foot and then draw your line before placing your left foot down, it seems you are asking whether you should put the toes or the heel of your left foot on that line; however, you have not described how to draw the line and using the two feet as the only reference points is circular.
Imagine you are standing in a horse stance, that is, with the feet parallel to each other about a shoulder-width apart and with the line connecting the heels being at a 90 degree angle with the lines connecting the heel of each foot to the toes of that foot. Your feet should look like
this. Imagine that in front of you is north, in back is south, to the left is west, and to the right is east. Now turn your right foot out, pivoting on the heel, until the foot is at a 45 degree angle and therefore pointing straight to the northeast. Now slide your left foot one inch forward (i.e., directly north). Your feet are now in position for an unlikely, but acceptable bow stance. (I am ignoring how to meet other requirements, such as the proper weight distribution between the feet and how much to bend the knees.)
Now lift your left foot and slide it further straight forward/northward, putting the heel down where the toes just were. Your left foot is in a new position, but this position is also an acceptable bow stance. Now repeat this last procedure one more time, moving the left heel forward to where the left toes just were. This is still appropriate positioning. In fact, you could slide the left foot forward indefinitely, except for the fact that if you go too far, you will be unable to keep your balance and move according to other Tai Chi principles.
Another way to look at this is that when you are in a proper bow stance. Your heels should be placed at the opposite corners of an imaginary rectangle. Let's call the bottom "east-west" line of the rectangle the width, since in most people this will be shorter than the "north-south lines." At the top is the other parallel line. The other two north-south sides of the rectangle at 90 degrees to the bottom side represent your optional stride length. To form a proper bow stance, the width of the rectangle must be shoulder width, but there is no requirement as to the length. This means there is no special line that can be drawn between the back heel (or toes) on one hand and any point on the front foot on the other to tell you your stance is correct. It depends solely on how long you have decided to make your rectangle.
Try looking at
this diagram. It is correct for our bow stance except that we have no length requirement and so you should ignore the gray paw print and the text that says: "one of your own foot lengths goes here."
Even though I say there is no length requirement, there is a length that should generally result from how you step. Generally, it should be controlled by how much you bend the knee you are using to bear your weight during the step. If you then step out with the moving leg stretched out, there will generally be only one correct place for it to land. This relationship will vary tremendously, however, according to how much you straighten the knee of the leg that is stretching out. From the outside, looking at another person, I would easily accept as much as a variation of one foot. From the inside with respect to my own movement and feeling, I would vary the placement only by two or three inches at the most without feeling I was violating some other principle.
If all this still is not clear, try looking here for the real authority on our style (i.e., Master Yang Jun)
at this time interval and
this time interval.