I don't think they ask to calculate it but it does help to have an idea to answer questions.
Also, it not hard to get a rough estimate-- just double the Na concentration and add 10 (provided Glucose & BUN are normal)
If Glucose & BUN are NOT normal then double the Na concentration and add 1/5th of Gluc & 1/3rd of BUN.
Cool, thanks transposony! Ya I kind of know it but equations just never stick in my brain, so pretty much the big equations (like all the pharm ones haha) are on my "dump" sheet that I practice writing up so do during the first few minutes of the tutorial so that I don't mix em up/forget them in the test.