This is how the back of my pants usually look, RTW or sewn-by-me. I've been calling it the X-wrinkles. The X-wrinkles are wrinkles and folds that radiate from my inner thigh to knee at the back.
While a fisheye dart (FED) seemed to cure most of the problems, it never quite resolved all of the X-wrinkles. As my pants fitting continues to evolve, I think I've found my solution by addressing the X-wrinkles as two separate fit issues (high hip, full inner thigh) and altering for each by incorporating a "wedge" to add more fabric length where it's needed. In other words, adjusting where the X-wrinkles are pointing to.
If the FED works for you, keep using it! The adjustments below and/or here (Minott) and here for full inner thighs are what I think works better for me.
The photo above shows my FED pattern (white) laid over my wedge-altered Simplicity 4068. You can see there is a leg "shift" difference between the two.The altered Simp gives me more fabric at the upper inseam and at the high hip. (Pardon all the lines and scribbles on the FED pattern -- I was experimenting.)
In addition to the FED alteration, I had also been altering for full thighs by adding to the crotch points. Yet, I would still have rear "crotch smiles." So, I would add more. And still I would have those crotch smiles. Clearly, this wasn't working.
So I dug out the fit books and realized that while I have full thighs, the fit alteration I really need was one aimed at knock knees because my full inner thighs were mimicking the results a truly knock-kneed person would see. And the light begins to turn on …
I found the inseam wedge alteration in Fast Fit (Sandra Betzina) and coupled that with another wedge at high hip to add the extra length needed to go over that area. It's almost like splitting the FED into two, which for me is what I need because each side of what would be a FED needs a different size wedge.
Below is my altered Simplicity 4068 pattern.
The red lines are the wedges. The upper wedge is for high hip.
(I added the high hip wedge lower on the front than I really wanted, but I was trying to keep the pocket area unchanged. Because the first sew of these pants resulted in a bit of poofiness at the front crotch, next time I will add this wedge higher and redraw the pocket.)
The lower wedge is to add length at the inseam.
After my initial pin-fitting in fabric, I also incorporated the alterations shown in yellow and blue.
The yellow lines reflect (1) removing hip width, (2) lowering crotch depth front/back, (3) lengthening the rear dart, and (4) returning the crotch points to the original pattern lines because I didn't need the extra I added. (Truing the grainline is also shown in yellow but I did this before cutting the fabric.)
The blue line at the back crotch curve shows where I deepened the curve.
The black lines are original pattern seam and grain lines. You can see that these alterations do not throw off the grainline very drastically.
This is the result, standing with my legs together …
… and with my legs slightly apart:
And this is how I usually stand, one knee locked, one bent, and one hip higher than the other. Yeah, not every stance is a perfect view. Once you move in real life, you *will* have wrinkles. LOL!