Understanding what's fitting varies with different cultures and situations, not just universal standards. Although loud cheering is seemly at sports events, it might not be in a ballet theater.
Pro Tip 2/3
Social Settings
Use 'seemly' when discussing behavior in social situations which require a level of decorum or propriety. At the formal dinner, her seemly attire was noted by all.
Pro Tip 3/3
Ambiguity
'Seemly' can be subjective; what seems fitting to one might not to others. Use when the consensus is clear. He found her jokes offensive, but the rest of the crowd thought they were perfectly seemly.