Sorry I can't offer advice on the timing.
However, be aware that a lot of flights to Koh Samui on Bangkok Airways are on propeller planes ... they have introduced jet planes on that route since we were there last but they still use props.
If you are any way concerned about flying in a prop plane then choose the flight carefully. Have to say our early evening flight from Koh Samui was very very scary... we circled Bangkok 3 times before we landed in a storm and passengers were white faced when we got off, even my husband who normally loves flying was terrified (so he told me later)
Cheers Geri