Simple mathematics tells us that if the difficulty increases by the same relative amount every time, the total amount of dividend paid out by MINING over its entire lifetime is:
14 * X / D
You are using 14 days here for the length of the difficulty period, and that is not correct when difficulty is increasing. Therefore your formula will overestimate the value of MINING.
While it may appear like that, do the math, sum up the geometric series, taking into account that each cycle only lasts for 14 / (1 + D) days and you'll see the simple expression that I mentioned previously emerge.
edit: If you want to deprive yourself of the fun of deriving it yourself, you can check
https://bitcointalk.org/index.php?topic=228327.msg2493461#msg2493461