You cannot make the glass turn the corner too sharply. An outside curve will generally work well; however, an inside curve is likely to put a lot more stress on the sheet. Rather than just asking how hard to press the glass cutter, you should ask yourself if it is possible to cut the curve in one score or if you need to remove the surrounding waste glass in stages.
Look at your pattern piece before you begin to see what you are dealing with. If the arc of the curve is wide enough, the glass is going to be able to follow the score line easily. But if you have a narrow hook or a deep inside curve, you are going to want to be careful that you don’t cause the crack to travel the wrong way. This is particularly true for early stained glass projects, when you can benefit from patterns with big curves that allow you to get used to the direction of the cutter without having to take a risk on every single break.
The score still needs to be made in one smooth, uninterrupted motion. The trick is to move your entire arm in the direction of the score line and not just your wrist. If you find that your cutter is not following the curve as well as you would like, do not go back over the line, because a second score can create competing fractures just like it does on a straight cut. Instead, use the piece to inform how you go about your next attempt, and cut another test piece using a gentler curve on your scrap glass.
It may also help to draw three different curves on your practice scrap glass, ranging from a gentle arc to a sharper outside curve to a slight inside curve. Score and break each of the curves and see how much resistance you feel as the curve gets tighter. Then remove the waste glass surrounding the inside curve bit by bit using grozing pliers or a glass grinder, instead of trying to break the piece out in one piece.
Grinding is best done once you have made a fairly close cut; you shouldn’t expect to grind out an entire curve that was cut too far outside the pattern line. If you leave too much glass around every curve in case you can’t make a clean break, then you’ll have to grind off too much glass and may end up changing the dimensions of your pattern piece. Once you have cut your piece, dry-fit it to see where you actually need to adjust it. It’s much easier to fine-tune the outside curve or corner of your piece than to grind the entire edge.
Cutting curves is a lot less frustrating once you stop worrying about completing every curve line in one score. Some curves will be easier to achieve in one score, while others may need the waste glass removed in stages before grinding the final edge. Whenever a curve begins to feel as though it needs a lot of force, it is probably a good time to rethink your cutting strategy rather than press harder.