The STU calculation is very complex and hard for people to understand, so the calulated value in USD is used, based on the Steem price average held by the witnesses. This is always days behind, so no calculation is perfect.

Yes, at this time, if your post does not exceed $50.00 USD in payouts from bidbots, it will not be commented on. This is not the amount you spent (or someone else spent), but what was received from the bot.

Blessings!