There are two things to consider with any selection method...

(1) The best horse in the field does not necessarily always win the race, in fact I would suggest that the ratio is a lot lower than what one would think. So if your selection method picks the best horse on class, form, strike rates, barrier position etc ; you may not necessarily get the winner even though your research says that it is.

The varying reasons for this are luck in running, suitable pace, barrier draw, the jockey riding the horse exactly to the very best to suit the horses method of running and being able to give it the run of the race.
The list goes on.

(2)No rule should be set in concrete.
For example you may include any horse which has a 50% place strike rate and discard any below this. However, there may be another horse in the field which has been running below it's best because of bad luck, bad barrier positions etc and comes out and wins a metro race by 5 lengths. Now that horse may only have a 20% place strike rate and so it doesn't qualify - and yet it should well be included.

So my summary is this...

You must have rules, and guidelines. But each race is different, and each horse is different, therefore you must be objective and bear in mind your rules, but look at what in the field may be an exception to this rule.

Just my thoughts.