@Mia Chloe,
OP said the missing string consists of capital letters and numbers in his first post, Findingnemo's calculations seem correct. You are correct that if the op did not specify this condition, the number of possibility's would have surpassed Findingnemo's calculations.
I noticed that but I'm a little skeptical about what the OP said because I've haven't really seen a case of all upper case only in a private key.
Do you mind making a little explanation how feasible it is.