I assume you realize that the screw is probably only a "set screw". I.E., it is not strong enough to tighten down to put sufficient pressure on the leather. Because while the set screw MIGHT be steel, the threads it is tightening against are just cast iron - not a very strong (easily deformable) material. Instead, you use your stronger hand - possibly with the help of a a pliers with a lot of leverage - to squeeze tight on the leather, and use your weaker hand to tighten the screw. There are other brands and models that look like they are real steel, - but I haven't personally tried them.
One problem with most of these tools including yours is that the leverage works the "wrong" way - your hand moves a shorter distance than the ball and ring. That means you are putting less pressure on the spot than your hands squeeze with. I don't understand why they are made that way, though of course it would take more metal to make the handle that long.
BTW, the boot press at a good skate shop - or a good downhill ski shop - can manage much higher pressures.
Also, it helps a lot if you warm the spot, inside and out, before stretching, with a hair drier. Up to about 100-140 degrees, but be careful, because heat can also undo the stretch on other spots. In effect you are doing a spot "heat mold". Theoretically you could take it up to the temperature that your boot model is designed to be heat molded at, but be really really careful not to heat other spots. And moisture helps - though I admit it isn't good for the leather. I have sometimes used a bit of "shoe stretching oil", but it doesn't help much. If you have doubts that your laces can take the heat, take them out first - in fact, even if not, that makes it easier to work your punch all the way down to near your toes, if that is needed.
Again, you should do this to get as tight as you can. Wait an hour or so. Repeat, tightening it a bit more. And again. And again. Wait a day or so if you can. And tighten again. And let it sit some more.
Don't forget to put a thin soft cloth on the outside of the boot, if you care about leaving marks on it - but I don't bother. And it makes the stretch a bit less effective.
I have slightly more detailed directions at
http://mgrunes.com/boots/BootPain.html.