I use the recipe on the cream of tartar box too. But I don't cook it, I put everything in a bowl, boil the kettle and add the water boiling. Mix it up and it's done.
same here. a kinder teacher suggested doing it this way & it works a treat. her advice too was to go easy on the salt & add more if you need it. she reckoned that it was tough to come back from too much salt so better to be too little & add more!

ours lasts for ages - would definitely last for longer if i always remembered to put it away