Heyo, found a similar exercise for 3rd eye opening in Doug De Long's book on ancient teachings for beginners... the difference being that there's no mudra and the syllable used is "Thoh", rhyming with toe. (I'm having a pretty hard time with trying to say "Thoh" without losing the vibration of tongue against teeth!!) It similarly advises that this exercise be separated by a 24 hr period.
"Thoh" is for the 3rd eye
and the syllable to chant for crown chakra is "May" with concentrating on moving energy from the brow up to the crown, from brow - middle of brain - to crown.
His book says that "Thoh" need be done only once to open the 3rd eye but that "May" can be done repeatedly to enhance skills such as clairvoyance.
"Thoh" is for the 3rd eye
and the syllable to chant for crown chakra is "May" with concentrating on moving energy from the brow up to the crown, from brow - middle of brain - to crown.
His book says that "Thoh" need be done only once to open the 3rd eye but that "May" can be done repeatedly to enhance skills such as clairvoyance.