"I have always assumed that a .3 add-on is necessary"
Starting in (I believe) 1973 the IAAF began recognizing 2 sets of world records in events through 400 metres, hand-timed and fully-automatic timed. Prior to that all sprint and hurdle times were rounded to the nearest tenth. With the advent of the transition to auto-timing, the accepted practice for a few years in the mid-1970's was to add 0.24 to times for events up to and including 200 metres, and 0.14 for 400 metres, when "converting" hand times to auto-times. It wasn't uncommon to see converted hand-times lumped in with auto-times in performance lists, with an asterisk identifying the converted times. For example, a 10-flat would be *10.24 on a list, or a 44.8 hand-timed 400 would show up as *44.94. This weird practice meant that we witnessed double-world record performances in the women's 100 just prior to the Olympics in Montreal, when one West German (Annegret Richter) tied the world (hand) record at 10.8, while in an entirely separate meet a second West German (Inge Helten) took sole possession of the world (auto) record with an 11.04, which Richter later lowered to 11.01 in the Montreal semis.