I may be misunderstanding, but are you saying that the dragon essentially starts completely off the screen, and then animates it's way onto and then completely across the screen until it is completely off again on the other side??
If so, than it's just dragonwidth*2 (for each side) pixels + screenwidth ( which will give you the pixel count of the dragon completely off on each side of the screen, and size of the screen itself)
then take the result and divide it by 200 to see how many pixels the dragon should traverse per frame over 200 frames.
or, more accurately
(dragonwidth + screenwidth + dragonwidth) / number_of_frames_used_in_crossing
will give you how many pixels each frame should advance the dragon by.
If you already know how far the dragon advances in each frame, and you're just trying to figure out how many frames it will take to get from one end to the other it's
(dragonwidth + screenwidth + dragonwidth) / number_of_pixels_advanced_per_frame
will give you how many frames are necessary to move the dragon from one end to the other, divide that result by the number of frames per step(20) and you'll have the resulting number of steps necessary to get the dragon completely off-screen from one side to the other.
It is also quite possible that I have completely misunderstood your question and am visualizing this improperly.