Calories pre-cooked vs cooked
You can easily calculate a maximum calories difference, and not so easily approximate an actual difference.

If you measure the weight of your meat pre- and post-cooking, you can regard the difference as the maximum amount of fat that has been lost from the meat.  Multiply that difference by the number of calories per same unit, and you will have a good idea of the maximum difference in calories.

If you want a more exact number, catch all the drippings and try to determine the percentage comprised by fat. You could boil your drippings for some period, and what is left should be (mostly) fat.  That seems like an awful lot of work, but you may be able to do it once and decide that for your purposes whatever you came up with on that one calculation is good enough to apply universally.