Yes, the only way to legally log safety pilot is to do it under the hood. There's no real rule for taking tenths off... essentially, if the pilot under the hood logged .5 of simulated instrument, that's how much safety pilot time YOU will log.

I've heard of plenty of guys who split a twin, take a trip, and don't do any hood time. They split the time evenly, each logging half the trip as safety time, and the other half as PIC. More power to them if they can find a way to explain all that safety pilot time in an interview...