Pretty sure I've read instructions from Trangia themselves that say it should be about three quarters full when settled but I've always just gone for either how much I think I'll need or (if I have a seal-able lid with me) whatever is in there at the time from the last burn.
Alongside insulating it/pouring meths in the rim around the burner to heat it up if you still have the seal in the lid then you can simply fill it and keep it in your pocket so your body keeps the meths warm. I have found this is the best thing aside the insulation and the meths in the rim (not on the floor please, not good to burn the ground unnecessarily) to help the burner bloom.
Alongside insulating it/pouring meths in the rim around the burner to heat it up if you still have the seal in the lid then you can simply fill it and keep it in your pocket so your body keeps the meths warm. I have found this is the best thing aside the insulation and the meths in the rim (not on the floor please, not good to burn the ground unnecessarily) to help the burner bloom.