It is pretty simple really at the end of the day Andy. Work out your maximum load now and in the future and when the generator is pretty well loaded, decide what the largest load you have can be load dumped on the generator and the type of load. That may be a large motor, or a UPS, or whatever. At the end of the day the calculation HAS to take into account the largest load that can be dumped on the generator at any time. I know of many generators where the first step can be a maximum of 40% and the second step 60%. I know of others where the first step can be 100% - oversize the generator. The generator guys need to know maximum load, maximum possible load dump at any time, maximum possible first step load dump etc etc. At the end of the day they just oversize the alternator/engine to suit the application. Just costs more bucks.