If your EC has gone down, you top off w/ full nute- your plants are feeding more than transpiring.
If your EC has gone up, you top off w/ water until well below desired EC- otherwise you'll go high pretty quick- your plants are drinking more than they're eating.
You adjust the strength of your top-off solution to maintain a consistant EC in the res, or at least a manageable swing. Better to go low than high.

How often you change the res depends on the biomass/volume ratio. If you don't know what that means, do a full swap weekly.

Fertilizer is cheap compared to product- don't f%ck yourself over a couple bucks.

I've had rigs that would transpire 14 gallons a day- 14g is not very big at all.