A Reference Putt should be part of
your preparation before teeing off.
To judge how hard you must hit your ball requires an understanding of how fast the greens are running. You get this information pre-round by using a drill on the practice putting green.
It is one of the ways for you to evaluate the speed of the greens prior to the start of your round.
Most putts have some break to them as pin locations are set on a slope so that water doesn't pool around the hole.
The amount of break to factor in for each putt depends on the degree of slope, where your ball is located, and how fast it will be travelling when it reaches the hole.
The optimum speed must allow your ball to hold its line, yet be going slow enough for the hole to capture it. If your ball is going too fast without sufficient time to drop half its diameter before it hits the back wall, it will lip out.
To judge how hard you must strike the ball, you need to know how fast the greens are running that day.
You need a reference for feel that you can rely on. Some courses may post a green speed reading for the day, but this is only a number.
Hopefully the practice green is similar in green speed to the other 18 greens on the course. For it is here that you will tune in your brain before the round.
The purpose of the reference putt or core putt is to give you an internal gauge as to how hard you have to stroke a putt to travel a certain distance.
Most weekend golfers when they arrive at the practice putting green putt to different holes to see if they can hole their ball. Few putt to the fringe to fine tune their distance control. Even fewer use a reference putt to gauge the green speed.
Getting a feel for the green speed is far more useful that just putting repeatedly to a hole. In fact if you want to putt for accuracy it is better to putt to a tee peg and not the hole.
Here is how to practise a reference putt:
The sole purpose of the above practice is to get a feel of the green speed for the day. That feel will translate into the size of your backstroke on the course without you having to think about how far back you should take it.
Never consciously think about the length of your backstroke during a game. Just let it happen.
You simply can't putt successfully by numbers.