You might not want to get too hung up on sound deadening material. Anything is better than nothing and stick on felt works just fine. Most of the noise from a typewriter comes from the paper being slapped between the platen. A new platen will give you more quiet sessions. Any felt added will keep out any high pitch resonant echos but it's not going to do a whole lot for the overall volume you will experience. A good typing mat like wool, and a new platen are by far the two optimal solutions for noise. Felt can be a bonus but unless it's a complete tin can rattle trap, the difference between 1mm and 3mm felt probably isnt going to rock your world.