How do I calculate needed pixel clock frequency?
There is more pixels in the video signal than just the x * y * refresh_rate would imply.  Back in the bad old days of CRT's, it would take time for the electron beam to move from the end of the current line to beginning of the next.  Likewise, it would take time for the beam to move from the bottom of th screen to the top for the next frame.  The time it took was built into the video signaling standard, and essentially took the form of pixels that are never seen.  

Without knowing the info on these blank pixels (sometimes called "overscan"), you cannot calculate the pixel clock required.

The best way to figure this stuff out is to Google for the approximate video standard you're trying to do, or look at the datasheet for the screen you are trying to drive.