https://forums.backpack.tf/topic/6033-rules-for-unusual-price-suggestions/

Rule - for all unusuals above 10 keys, please round to the nearest whole key

Use the value of keys on the site at which the item sold to determine the key conversion at the time of the trade. (i.e. use the SCM value of keys on SCM sales, use the marketplace.tf value of keys on marketplace.tf sales, use the opskins value of keys on opskins sales). For most transactions, this means you can use the key value at the exact date of the sale.