chibears, i avoid the whole issue by exchanging a lump sum at the cambio, then using only $J for my walking-around money. i never convert in my head, cause i don't care what the US equivalent is. i carry just local currency in my pocket, and use that for everything. for me, using only one currency is so much easier and less cumbersome than thinking about conversion for every little thing i buy during the day; it just doesn't matter to me whether or not i know the juice and coco bread in my hand for 250 $J is the equivalent of 2.85 $US. i have a pocket full of $J and just use it.
it's only for larger expenses, like buying a piece of artwork or a fancy dinner out, that i use the rule of thumb in multiples of 10; so i have some idea of what i'm spending in $US. Once you're there for a few days, it gets easy.
