I shoot both and stand the same with either one. Not to be picking apart your form, but in your picture you seem to be leaning back and your elbow looks pretty high to me. I don't know how that would affect your distance estimation though.
I have recurves and a longbow. I have them tuned to hit in the same spot. The only difference is the longbow has a flatter trajectory, so beyond 20 yards, I begin to notice a difference. The difference is negligible inside of 20 yards. Is your longbow much higher or lower in draw weight than your recurve?