(Bloomberg Opinion) — After the sudden collapse in equity markets in mid-March, the assumption across the board was that not only was the long bull market over but that a bear market had started. But wait, no: Now there’s a new bull market because some stock indexes briefly rose more than 20% from their March 23 lows.If this all seems a bit silly, it’s because it is: The 20% yardstick for determining the beginning or end of market cycles is simplistic and blindly mechanical. Indeed, 20% doesn’t do much to help you figure out where to put investment capital to work.It bears repeating: The average investor should be working from a long-term plan that won’t blow up because of a short-term disruption caused by issues