In every cocktail you ever make, the measurements are extremely important. That being said, you can look at five different cocktail books, and find five different measurements for something as simple as a Screwdriver. In the end, the measurements in a cocktail are to your taste. There might be an official version of the drink, but feel free to experiment, and find the balance you like. Of course, the standard measurements are standard for a reason, but that doesn’t mean you can’t find a version you like better.
Be careful, though, because if you vary it too much, you might wind up making something completely different; the difference between some drinks are the measurements or glassware (which affects the measurements).