How can I calculate how much my desktop PC costs to run for an hour?
No, your Windows Task Manager will give a really really bad estimate of how much power your computer currently consumes. Because, it will show you how "busy" a processor is. Moving memory around takes different amount of power than to calculate things. 

There's different ways to make a processor busy, or look busy. And they all take different amount of power. 

The greatest power hog in your computer is probably the GPU, which as far as I know you can't really see in the Windows Task manager.

And your equations are correct.