I haven't had to mess with IVs at the old folks home. We are slightly more medical than a lot of places but not quite that medical. With doses and medicines, I know I have trouble so I triple check and, if I have even the slightest shred of doubt, I ask someone to check. The good thing is that most medical calculations are actually very simple. That's the trick with most stuff in nursing: everything is geared towards being as simple as possible because it's safer that way
I also understand basic concepts like, for example, if I'm working out how many ml of oxynorm to give someone and it comes out as 2ml, I know something's up. No-one is getting more than 0.7ml on our ward at a time and that's a fair amount.
That's the main way I cope with it, I know I suck at it so I'm ultra careful. Sometimes more careful than people that don't have a problem with maths. Also, it's not that I'm incapable of doing it, it's more that I'm just really slow and sometimes take odd routes to get to the answer. My medical calculations teacher was like "Oh yeah there is absolutely no question you have dyscalculia. The way you work stuff out is weird."