transactions – How to Predict Transaction Hash Before Submitting

I'm trying to figure out how to get the transaction hash for some transactions. I don't have the hashes in my data; all I have is approximate time (to the minute), size, and token. I think I can get the raw transaction hex. I want to get the transaction hash. I looked at this post: How to get raw ethereum transaction hash but when I hashed the raw hex myself I didn't get the same hash as the poster, so I don't know what I'm doing wrong. I hashed it with and without the '0x' before thinking that that was the problem, but it wasn't.