How to calculate a number of blocks (air) from surface block to player (for y coordinate)

here's how I would approach the problem:

first, subtract the player y value from the surface height. if the value is positive, start at the player, and make a loop that checks each block between the player and the surface. every time the loop detects air, add one to a variable. if the value for the initial subtraction is negative, do the same thing except starting at the surface and going up to the player.