If I know my paycheck is going to be $1000, and I know bitcoin is $12, it's simple division. If I know dollars are 0.08 BTC each, it's harder to think about quickly (at least for me, YMMV).
OK, lets take that example. You received 1000 usd, that is 1000 12 usd/btc, which is, umm, where's my calculator? Starting over: You received 1000 usd, that is 1000 * .08 btc/usd, which is 80 btc as your income, did that in my head.