"I just let it slip today on 14, 15 and 16, but it was good to make a birdie on the last to finish on a good note."
McCall, who finished 13th in the individual standings of the Eisenhower in Turkey in October, is looking to go low in the final two rounds to put the pressure on the 14-year-old Chinese leader.
"I just need to get doing what I am doing," McCall said. "I feel like I am playing really well and I feel like there is a low number out there for myself. It's out there I just need to be patient. Ten shots over two rounds is still gettable."
McCall said a key to staying in touch of Guan was managing the searing heat in Thailand - a stark contrast to the conditions he left in Gore.
"When I left home it was snowing - this is a bit tough for me. I had the umbrella out and I have never had the umbrella out when it is not raining."
Fellow Kiwi Daniel Pearce, who opened with a four under par 68 to sit second, carded a one over par 73 to slip back to tied 10th, while Tasman golfer Blair Riordan is tied 12th and Peter Lee is tied 33rd.
New Zealand amateurs have previously come close to winning the Asia-Pacific Amateur and earning the right to play at the Masters. In 2010 Peter Spearman-Burn finished fourth, and in 2011 Ben Campbell when one better.