Okay, so you’d add a Single Relation column to your Categories table, that joins the Category Name in that table to the Category Name in the Items table.
Then create a Lookup column that fetches the value you need from the Items table via that relation.
You can then use that second column in your math formula.
But your table structure still looks a little odd to me - what happens when you have multiple users, how do you know which bets apply to which users?