Frequency variation with small changes in position is normal in the high frequencies. If all you have is the 3.6 kHz dip, you're doing very well. The front and side wall spacing you've calculated should be working nicely for you. I doubt half way into the room will improve much. It will change your measurements, but will it improve? Only your ears can tell you that.
Unless I've done my math wrong, you're sitting 10' 8" from the speakers. You might try moving your listening position up 1 or 2 feet from the back wall (2' would put you at about the equilateral triangle position). This will increase the direct to reflected sound ratio and should improve both transients and claritynot dramatically, but noticeably. You can experiment with toe-in from there. Sometimes a very slight toe-in is all it takes. Give yourself plenty of listening time with many different recordings. It may take some days or weeks before you get it settled.
Unless I've done my math wrong, you're sitting 10' 8" from the speakers. You might try moving your listening position up 1 or 2 feet from the back wall (2' would put you at about the equilateral triangle position). This will increase the direct to reflected sound ratio and should improve both transients and claritynot dramatically, but noticeably. You can experiment with toe-in from there. Sometimes a very slight toe-in is all it takes. Give yourself plenty of listening time with many different recordings. It may take some days or weeks before you get it settled.