Selecting spring manually is fine but care has to be taken against using this haphazardly.

By selecting spring manually you are asking spring to take some load which is not real.You are asking spring to take some load, say 5000 N load in ope, so that your nozzle loads are within allowables. Then with spring rate of 100 and disp. 0f -5 mm, we get sustained load of 5500 N.
If the actual sustained load for the line at that point is more than 5500 N then the spring's ope. load will be different and will not act as we have designed it on paper.

So it is always good to let caesar decide the spring load for us. The sustained load should be such that the spring deflection in that case is as close to 0 mm as possible. In caesar and on paper we can make the
spring work as we want it to. But on the field it will work as it wants to. Hence tweeking the spring on paper, though useful, is not a great idea and enough care has to be taken.

Another advantage of having sutained def. close to 0 mm is that the installation of spring is also easy. The unlocking of spring will be easy too.