find and chmod

[gnomek]

find . -type f -exec chmod 660 '{}' ';'

This command changes all files in a directory to 660. But how it should look like when I don't want to change all files but only those that have 644?

Or even better how to change all that don't have 660 to 660. The problem is that I don't want overwrite those that already have 660 because it would last to long.

[kubicle]

find . -type f -exec chmod 660 '{}' ';'

This command changes all files in a directory to 660. But how it should look like when I don't want to change all files but only those that have 644?

find . -type f -perm 0644 -exec chmod 660 '{}' ';'

Or even better how to change all that don't have 660 to 660.

find . -type f ! -perm 0660 -exec chmod 660 '{}' ';'