That's quite nice and simple
It's about the way I do it, I can add that when I do it I also breathe outwards slightly different to get the differences in sounding between the two, also I am much better at growling than purring
Well, basically for the growl I breathe outwards to gargle much harder and open my mouth quite wide, trying to mimic a the expression of a growling dog.
Unlike you, I however either have my teeth wide open, mostly to expose my sharp and fearous fangs, or I semi-close them, pushing my tongue against the top of my muzzle
For the purr I close my eyes and gargle very softly, nodding my head down but raising it upwards during the purr, yet being a fox and not a cat, it's not that common for me to purr
If anyone is interested in barking I can also recommend these videos:
It's quite well explained and she does examples of it herself. It did help me improve my skills at least
How To HowlHow To Bark