That's why I laid it out the way I did with the comments - so it would be easy for you get the separate feet/inches values if you still wanted to go that way.
[edit] Look more closely at my code - you require two substring_index()s to extract the inches. The inner to get the string before the final " and the outer one to get the string after the '
SET feet = substring_index(@height, '\'', 1) * 12 ,
inches = substring_index(substring_index(@height, '"', 1), '\'', -1)