It bugs the hell out of my kids, but I love saying it:I don't do mental math well at all. I was never taught, and when I taught myself how to do it in order to teach my little afterschool Singapore Math class, I didn't practice it enough to learn it well. (I think Saxon Math has some mental math exercises, right? Or am I confusing the two curricula?)
The fact that you're asking for a calculator is the surest sign that you shouldn't have one.
I have seen far too many kids so clueless about basic computations that they can't even recognize an answer which is off by several orders of magnitude. They add two numbers and get a smaller number. No clue.
Now that I know a bit about it, I'm a big fan of mental math.