I'm surprised TC said that. It will be stamped on the barrel if it's a 1-66 twist. If it isn't it will more than likely be 1-48 twist.

If it is a 1-66 twist it will shoot a patch round ball well, but probably won't be good for conicals.

The 1-48 twist will be good for a patched round ball (PRB), but also good for conicals like the TC maxi ball.

The .54 cal PRB will be perfect for deer out to about 100yds. A powder load around 80-90gr will be good, but you need to shoot different loads to see what the gun likes.

Does the gun have brass hardware? How long is the barrel? Can you show us a picture of the gun?

A flintlock needs to have a lot of things perfect, or you'll get delayed fire. So, be picky about all the little details of loading, and you must repeat everything exactly the same everytime to have reliable ignition. A flintlock is old technology, and requires more care to get right.