@Green it's a nice solution introducing a bit of psuedo/unscienced science which makes it easier to integrate/play around in a plot. Does not necessarily explain or imply why newer timemachines are able to go back further though(?)
(also it's not my wuestion, i just hijacked it with my mumbojumbovoodoo answer mwahahhahaha)
@dot_Sp0T true. Well, newer machines have the benefit of refined branch point precision. There are always lots of branches at all times. The trick is the ability to identify them, which benefits from experience.