what's the essence of calibrating speaker distance?
-different room sizes/configuration will have different speaker distances to the audience, so in general you calibrate the speakers to give the same audio level to you (or to where you'll be sitting). If you will be watching mostly near the back, or near your surround speakers, without calibration your surround speakers will sound louder that the ones in front. If calibrated, all speakers will sound equal as if your in the center of the room.
i mean my huge impact ba yun sa sound quality ng speakers? Yes and no. Audio quality is by preference. Audio level calibration will have a huge impact if you're really off the center position of your system. Some audio processing of AVRs muffle the audio quality especially if you pick a lot of area points for calibration. For me, I just turn it off and sit as center as I could. And maybe just tweak the center and surrounds to be a bit louder.
how to measure distance manually? what's my base point of measurement? Your receiver should give you the measurement after calibration. Just rough check it using a tape measure if you have one. Your should measure from where you will be watching, and mostly that's where your couch or sofa is.