The stuff about nutrients are questions I don't feel comfortable answering too specifically, because I've never done DWC and it's been a LONG time since I've used chemical salt-based fertilizers.
Adjusting EC levels is as simple as adding more fertilizer (specifically chemical salts) to drive EC up, and adding more (0 EC) water to drive it down. Also, you need to know that the plant's needs with specific regard to EC levels changes as it grows and according to what phase of growth it's in. For example, a vigorously growing cannabis plant in vegetative phase may need an EC as high as 2.0, whereas a girl late in flower is just sipping nutrients, so an EC of around 1.5 or under may be called for. It's also very strain-dependent. And, to further complicate things, you can't go just by EC, you need to be able to respond to actual nutrient requirements, another example would be how The White really demands so much more calcium than just about any other strain I've run.
Don't worry about ppm or TDS, again, they're just conversions or calculations based on EC.
As for when you check EC, yes, you do it when you mix up the nutrients prior to feeding. That's also when you check pH first.