The simplest formula to remember is "10% +2" ie ten percent of the
leader's pipcount plus two pips. For example, if I'm leading 70 pips to
79 pips, I'm at 10% +2. Round up if 10% +2 is not a whole number.
10% +2 is the trailers point of last take. If you're more behind than
10% +2, then drop, if you're less behind than 10% +2, it's a take.
The leader should double when he's within three pips of 10% +2, and
should redouble when he's within two pips of 10% +2.
For example, if I'm at 70 pips, the 10% +2 is 79. If you have 79 pips
it's barely a take. More than that it's a drop. Less than that it's a
take. I should double if you have 76 pips or more and redouble if you
have 77 or more.
10% +2 is known as the Trice number.
It's applicable at pipconts of 62 or more.
Below 62 pips, the Trice number is calculated by (pipcount-5)/7 rounding up as needed. Use the same criteria to arrive at the double take decision.
You can refine it further by using effective pipcount instead of the