Middle Split Help

My front splits are almost perfect, but I feel like I am stuck on the middle split. I stretch at a minimum of 4 times per week and do stretches like the butterfly, pancake, holding the middle split while lying on my back, and holding the split with one leg bent or a full split. I try to be cautious because sometimes I feel a slight discomfort in my left knee. Does anyone have any suggestions for a stretch and is there a good way to align your legs to minimize the discomfort in the knees?

(As for my progress, I can lower down until my body is about 6~7 inches from the ground.)

I prefer to train all splits with gymnasts on a line.

You may want to try stretching the middle split, froggy style. This means split from knee to knee, sideways, but the lower limbs will bend back so it focuses from knee to knee.

More than likely you have some issues with that knee that need to be sorted out. Tight this, weak that, etc. Start rolling it and explore to find out what is tight.

I guess I should spend more time doing the middle split in frog style.

Sorry for no direct links, but if you do a little searching in this forum, you will find posts discussing flexibility training and links to very useful websites.