Which tool for nutrient dosing?

It will depend on how big your reservoir or watering can is. I usually make up in a watering can and I use anything from 1ml-40ml. The syringes are best. Get a ten ml one and it will easily fit in the bottles. Another option would be measuring beakers. Some bottles like canna have a measured part on them. You should be using a ppm/ec/TDs meter which will tell you how strong your solution is. If and when you are using A&B for base only , you use the same amount of each and test the strength. All in all any liquid measuring thing will do like what you would see in a lab or science class. Beakers you pour into and syringes should fit in the bottle but let's say you have a 100ml one or bigger than doesn't fit in then what you can do is take the plunger out and put your finger over the hole and pour into there.