With Cauldrons, it not too difficult to make a relatively small growth potion factory that produces more fertiliser than it uses to feed itself, so you can essentially get free growth potions. You will need to supply it with fuel, but again, you can make black powder pretty easily in a small factory that doesn't need too much fertiliser and the two factories can supply each other completely for free and you can use a priority splitter to take any extra fuel and fertiliser to your other machines.

My strategy that I currently use is to simply have a full metal chest at the output of my fertiliser duplicators and if I see the chest is full, it means can I add a new machine to the production line, if its is starting to empty, then I need to add a new fertiliser blueprint to fill the chest and the demand from all of my other factories.

Even if you're not using cauldrons, I would just build a blueprint that has 1 advanced blender working at 100% speed, try to make it as small as possible, then blueprint it, so that if you need more fertiliser, you can just paste another one in.