If you are interested in pattern alterations try studying a pattern you already have. (Not all are perfectly drafted but you can still learn from them).
Usually your back pattern piece is larger than your front but by how much?
See how much deeper the center front point at the hemline is than your sides. Become familiar with how a hemline is shaped. Usually it's bowed so cf is a deeper by 1/2 inch to accomodate bustline. Maybe you need more.
Notice the slant of the shoulders and the width.
See how much ease is in your sleeve by walking your sleeve pattern along the edges of your armhole.
How deep is your armhole.
Compare it to one of your favorite best fitting tops that is of similar shape. Which size is closest to that top - is it several sizes?
You need to become intimate with your patterns and make them your friends by spending time getting to really know them.
Do this often enough and one day you will be able to just look at a pattern and know instinctively what needs to be altered...