I agree with both those approaches.
Also I ended up coming up with them myself, I didn't read them anywhere. Once I got myself in the right mindset, it came to me. I believe that's all you need.
You are starting out correct. You know you don't want to say "I miss you too." Why? Well cause we know the most attractive guy is not dependant, but also loves the girl he's with. How do you do that? Well those ways of doing things naturally squeeze themself out of you eventually, if you are keeping that mindset.
When you say "I know" you have to say it in a way that says you KNOW you're being cheeky

So if it's a text, put a

on the end of it, if it's real life, say it with a cheeky grin.
So you worked out how to not say nothing.. but saying "I know" is going to get old after a while. What is there left to do? You can play with the material you've been given. Asking her what she misses about you is so cool, as she tells you, she starts reminding herself of all the things she likes about you which makes her like you even more. And when you reward her with something like "That's so sweet!" you've given the mushy stuff in return, without actually saying "I miss you too."