For a whole pizza crust, it doesn't seem that surprising. Half of that would be 300, then.
It may sound excessive to you, but for the crust for half a pizza, 300 is not that much...
I am sure it's accurate. I took the recipe and entered the ingredients into a recipe calorie calculator website.
What you might could do is look on the internet for pizza places that publish their nutrition information and use that as a guide as well.
And sometimes you just have to face the fact that you're not going to get an exact number. You have to make your best guess and it will all even out over time anyway.
And now that you don't have to be perfect, you can be good.