Arriving at CDG

Hello everyone - I will be arriving at CDG on Saturday August 21st around 8am. I%26#39;m pretty certain that I won%26#39;t make the TGV that leaves around 9am or so - to Tours. So I%26#39;ve decided not to buy a rail ticket until I arrive at the airport.

Does this sound like a good idea?

I want to go to Tours from CDG but don%26#39;t want to wait for the 1pm TGV so I thought I would go to Montparnasse in Paris and catch a train to Tours there (I%26#39;m assuming there are more trains leaving for Tours from there than from CDG)

Does this sound like a reasonable plan?

Thanks

Derek