There truly is no method to my madness.
I buy the chia and flax as seeds, and then grind them up (separately) small portions at a time, to use when I feel like it. Sometimes I put both in, sometimes I just put one or the other.
For one batch of muffins, I probably add about a tablespoon total. I'd probably start with just one or the other. And maybe even start with a lesser amount and see if anyone notices. Mine never have. And I have a few pretty picky ones... BUT I've been cooking this way for a long time so I'm not sure if they have anything else to compare it too, lol