You can’t choose to do fractional damage to a target.

You divide the damage as you cast Meteor Shower, not as it resolves. Each target must be assigned at least 1 damage.

A tip on how this math works: To deal a total of 1 damage, X is 0. The cost is {0} + {0} + {R} = {R}. To deal a total of 2 damage, X is 1. The cost is {1} + {1} + {R} = {2}{R}. To deal a total of 3 damage, X is 2. The cost is {2} + {2} + {R} = {4}{R}. And so on.