I'm one of those guys that actually does "the math" when it comes to calculating doses. I'm OK at dimensional analysis, but I prefer a different method.... That being said, I very much will use apps on my phone, computer, iPad, etc to help me figure out how much of a given med I'm to give unless the math is stupid easy... like if I'm giving 50 mcg of fentanyl and it's a 100 mcg/2 mL vial or if it's 25 mg Benadryl from a 50/1mL vial... Just about the only thing I've memorized for doses is 1000 mg Ceftriaxone IM reconstitution. That's 2.1 mL of 1% Lidocaine and for a 250 mg dose, that's 0.71 mL as the concentration is 350 mg/mL when reconstituted that way.
There's a reason I use 1% lido to reconstitute... It's because I'm not a total a-hole. If I was, I'd just reconstitute with 2.1 mL sterile water instead.