I don't know anything about the medication (or beta blockers in general), but just wanted to say that 250 calories sounds closer to accurate for 5k than 450. The general rule is about 300/mile and 5k is 3.1 miles. So 250 might be a bit on the low side, but not terribly low.
Also, I'm not sure what you mean when you say you're staying a little under your goal, but if you're talking about calorie intake and your Spark range, I would recommend staying within that range, rather than under it. As long as all your info is entered correctly, that should be the right number of calories for you based on your activity level.
Congrats on losing what you've lost so far! :)