I think it’s a good idea to have some understanding of how much you’re actually spending on something. Unfortunately a lot of scams happen pretty quickly so I like to have a very basic conversion in my head of $1, $5, and $20 to the local currency. Basically it just makes it easier to quickly give me a guesstimate on how much something is and if it sounds about right. The point isn’t to have the amount spot on, but to just have an idea of the amount in whatever currency you’re familiar.
That said, it could still get confusing if you’re dealing with a currency that just seems to have too many zeros. For example, if $1 US is about 20,000 Vietnamese Dong, it could get confusing if someone starts saying the cost is “350” meaning 350,000. In the heat of the moment, just take a second and revert back to the basic exchanges. A quick guesstimate would put it at just under $20 so then ask yourself, does that seem reasonable? Remember if the person doesn’t have the patience to wait a few extra seconds for you to do the math, it’s probably not worth it.
Converting money isn’t difficult but if you’re not in the business of doing it regularly, it’s really easy to get flustered. If all else fails, just remember you can always decline and walk away. It’s probably best for your peace of mind to pay a little more and get currency that’s close than to get greedy and get taken. 50% of something is a much better deal than 100% of nothing.