As Ghengis says Hobbs usually works off a master switch or wired up to a generator so bias towards engine time rather than chock time. The CAP804 definition is when the aircraft first moved with the intention of flight, until it comes to rest at the end.
Personally I use decimal, but both methods are acceptable under the rules. Decimal is easier for someone from the calculator generation (I.e. Me).
As for rounding up/down, that is a grey area. (Probably on purpose)
Hope that helps.