Your cog, for that picture, is approximately in the middle of your cassette. With that same crank arm angle, you could compare the chainring's point of tangency for two other cogs: your smallest (fewest teeth) and your largest (most teeth). The smallest cog should make the point of tangency closer to the chainring's smallest diameter (better for higher speed), while the largest cog should make the point of tangency farther from the chainring's smallest diameter (worse for lower speed). Here's what is going on with your setting with the current cog. There are three points in the leg stroke. The first is when your foot is closest to you (leg position #1). The second is at maximum mechanical advantage of the crank arm angle (but middle of your leg's mechanical advantage as well as middle of your leg's stroke), which is where the line from the center of your hip joint rotation to the center of your pedal rotation is perpendicular to (90 degrees) the crank arm (leg position #2). The third is your leg's maximum extension, where your foot is farthest from your body and your leg is nearly straight. Your leg increases it's leverage from minimum at position #1 to maximum at position #3, but your crank arm leverage changes from zero at position #1, to maximum at position #2, to zero at position #3. The weakest leverage is at position #1 because both leg leverage and crank leverage are at their minimums, with both gaining leverage simultaneously from #1 to #2. At position #2, your leg has medium leverage, but your crank angle has maximum leverage, so it's not a bad position because the two balance out. At position #3, your leg has maximum leverage, but your crank leverage has decreased to zero, so from #2 to #3 there is also some balance. If you had the chain's point of tangency at the smallest chainring diameter when in position #1, you would have the best help from the elliptical chainring from #1 to #2 as the chainring diameter at tangency changes from minimum diameter (most chainring leverage) at #1 to maximum diameter (least chainring leverage) at #2. Remember #2 is where the crank leverage is maximum but the leg leverage is medium, so this is the best place for the least chainring advantage (due to largest chainring diameter at the chain's point of tangency). From #2 to #3, the chainring diameter decreases so that the chainring's help is increasing as the leg leverage is increasing (which at first seems unbalanced), but is actually balanced by the crank arm's quick, sinusoidal drop in leverage from maximum to minimum leverage from #2 to #3.
In your case, your point of tangency is not too far off at #3, so the effective difference in all positions will be somewhat small and you would have to test it to find out how much difference it makes and how different it feels. But for you, in position #1 going to position #2, the balancing effect of the chainring is delayed a little instead of being immediately available at #1. This is theoretically not ideal and might provide you with an improvement. At #2, the chainring's effect will be less than the potential maximum because the balancing effect is shifted a little bit to a position just after #2, where your leg advantage is growing, but your crank arm leverage is quickly decreasing. So the chainring's maximum disadvantageous leverage is being shifted toward the steep drop off of the crank arm's leverage, while the leg leverage continues to increase, in an attempt to compensate for the crank arm's quick loss of leverage. This is also not ideal, but is probably a small effect in your case. Going from #2 to #3, your leg leverage increases to maximum at #3, but your crank arm leverage goes quickly to zero, while your chainring's effect is shifted past #3, thereby not making the best use of your legs tremendous leverage while approaching #3, because your chainring's diameter is larger than necessary, thereby making the force required at the end of your stroke harder than necessary for the same torque output.
So, the bottom line is: A) Your setting is not far off from the theoretical ideal. B) The specific cog being used will affect the tuning by changing the point of chain tangency on the chainring at any given point in the rotation. Keep this in mind when tuning because a compromise will be necessary. Try it and post some pictures to see the effect of smallest vs. largest cogs on the chain's point of tangency on the chainring at #3. C) The way you have your chainring tuned now, the non-ideal effect will lessen as you shift into smaller cogs (fewer teeth), and vice versa. D) You will have to experiment to find out how much difference you can feel and what seems to work best for you. The best case would be that you could gain a little more advantage for free (due to effeciency), while the worst case would be that you would not notice a beneficial difference. I'm interested to see what you find.
