There are buyers at 21.25, indicating 21 is slightly too low

There's other places like Steam Community Market and Marketplace.tf which will have sales

https://marketplace.tf/items/tf2/1005;11

There's sales ~$50, keys are ~$2.05 which means the sale is ~24.4 keys when you round it

https://steamcommunity.com/market/listings/440/Strange%20Festive%20Huntsman

Sale for £39.83, keys are ~£1.74 which means the sale there is ~22.9 keys when you round it

So just briefly looking at that, something like 22.9-24.4 would fit the bill better than 21 flat.