While this is a fine suggestion, in the interest of being useful I will share that the (base) police strength of a single unit is equal to 0.01 * sqrt(size), and the police strength of one or more formations is simply the sum of police strengths for all of its units. This means 20000 tons of PWL infantry (strength 115) will be much more efficient for policing than 20000 tons of heavy tanks (strength 22 for 80 ton tanks), for instance.
Police strength is also improved by unit morale, so a commander with a GC Training bonus can also be useful, but I wouldn't use this fact for planning purposes as you can't rely on having commanders with this bonus.