A ph meter is a must if your growin in soil you can get by with just that, but if growing dro then you'll need a ppm and tds meter also... There's different methods of flushing like 3 times the water to your container size or the one i prefer is to flush with just plain water the last 2 weeks. If you don't have to many problems you shouldn't have to use the 3xwater method.
when you say 3x water method. your saying people just do that for there flush and thats it? (flushing with water the last 2 weeks) only water with water? or are you giving them alot of excess water each time? arnt a ppm and tds meter the same? Do you use the ph meter for testing the water that leaks out the bottom of the pot to know if youve flushed properly?
You can easily get by without any meters. I've never used them and do just fine. Just don't over feed and always make sure you have run off when you water and the ph should be fine. If you are using chemical based nutes you will want to flush the last 2 weeks. You can either use the regular amount of water you use during the flush....just no nutes. Or some people water extra. If you have a 5 gallon pot the rule is to use 3x that to flush. So 15 gallons of water every watering. It takes a long time to get that much water in a pot and I don't think it's needed. Just stop feeding the last two weeks and water a little extra each time. Just get lots of run off. Of course if you grow organic you never need to flush. There are even organic bottled nutes that are just as cheap and easy as FF or any other brands.
I am one who believes that excessive flushing causes the content to diminish. This is a theory but..... The way I finish is what makes my outdoor look like indoor, It is a very simple process and requires only two products. Kool Bloom dry powder and molasses. The last 4 or 5 waterings and is where the magic happens. This last phase should take no more than two weeks unless it is to cool to water often enough. Each watering needs to wait until the plants slightly wilt from thirst. I only flush once hard, twice if it is hot out when I am finishing(there is a reason I dont flush for weeks). Once the buds stop filling in they are ready to start finishing. I use kool bloom dry powder at twice the recommended dose for one watering. Then if that burns the tips of the water leaves I dont use any more nutes, if it does not i use one more feeding with a regular dose of kool bloom dry. The second to the last watering needs to have one tbsp of molasses per gallon and water heavy totally flushing the pot or ground. The last watering needs to wait until they dry out pretty good and then using high PH water, 8.0, flush with straight water. Once they wilt from thirst they are ready to cut. Flushing for a prolonged time can lower usable THC content as it causes the plant to start digesting its THC. I wont argue this but just say I know this to be true and some strains are worse than others about this. If you ever got some really good looking bud that did not have the kick it should considering its looks this is what happened, they flushed it to long. I have no testing to back it up but experience has led me to a short two watering flush, one week max, where the plants totally dry out between waterings.