Typicaly, when you drain the cooling system at the radiator it will remove about 90%, or so, of the coolant.. The thing you may want to use as a guide to whether you need a flush or not(I'd say not), is to inspect the coolant and what the inside of the radiator looks like.. If its relatively clean, dont bother flushing, just use the proper mixture. I prefer the pre mix coolant available at most stores now.. It comes mixed 50/50 with antifreeze and distilled water. It may cost a bit more, but its pre mixed(correctly) with "good" water that will add less contaminates into the system. It's also a good idea to check the voltage in your cooling system to determine if you have an electrolysis problem, just drop a volt meter lead into the coolant(radiator neck) and ground on the battery.. If you have more than .5V, you have electrolysis being generated,eating your engine(among other parts it touches) from the inside. Then I would definitly fllush it and check/change the coolant much more regularly.It may not be an issue with your tractor, but it is on cars, something I keep in mind though...