To use the schedule you look up the size of the thinnest piece of metal being welded. Looking across, you will see values for the correct amount of force, the proper weld time value, and the amount of amperage necessary to weld. If the current is doubled, the heat is quadrupled. Welding heat is also proportional to the time. Thus if the current is increased, the time can be decreased.
Pressure also has a direct relationship to the heat being generated.
Increasing pressure lowers the resistance, and hence lowers the amount of heat generated. Alternatively, lowering the pressure, increases the resistance, and thus increases the amount of heat being generated. Normally a short weld time is preferred to minimize heat losses trough radiation. There is no advantage to heating a large peripheral area. If you generate enough heat, you actually make the weld weaker by crystallizing the metal.