As a Manson gauge owner, I can personally see no way this gauge will render accurate results in a horizontal position. Especially since the maker's instructions clearly state:
"The Wall Thickness Gauge (WTG) is intended to be used in a vertical position, with the Indicator Block hanging down and the Base Block mounted against a wall or hanging from a bracket."
Nonetheless, Dave Manson ought to be the ultimate arbiter of proper use. Contact him at:
https://mansonreamers.com