I have a 1/2 acre pond
Ph is 6.8 and total alkalinity is 40ppm
Average depth is around 12ft

How much lime should I add?

You need to know what your waters hardness is.
Hardness is slightly different
You need at least 20 ppm
Here’s a link that tells the difference between alkalinity and hardness

http://aquanic.org/publicat/state/il-in/as-503.htm#Water\'s%20Physical%20Factors