writing - Usage of abbreviations in semi formal written conversation

The rule I follow with emails (which are, after all, still somewhat informal, even if you're writing to a business associate) is "would my dad or grandma know what this abbreviation means?"¹ Thus, etc., i.e., e.g., P.S. and ASAP are perfectly fine; FYI and perhaps BTW might be ok, but use with caution (and avoid in a more formal email, e.g. to your CEO or to a brand-new customer); and the rest are probably right out. (Especially CMIIW, YSVW, and YW: I don't even know what those mean.²)

^{1 Well, ok, so I always just use "dad", because my grandma never spoke a word of English, but you get what I mean.
2 Looking forward to definitions in comments. Yeah, yeah, I could look them up, but I'm too lazy.}