Best advice I can think of is to get a numeric drill set. That is instead of 3/32, 5/16, 7/8" etc, the drill bits are numbered 32, 57, 15 whatever.
Find the numbered bit that just fits into the touch hole, and then go up a size or two and using your portable drill, open it up to that size. Best done at the range on a slow day. Then fire off a few shots to see what sort of results you get.
As far as drilling too far into the barrel, I don't think that should be a problem, but to be sure, hold the drill bit so that it is at about center of the barrel/bore and put black tape around the bit at the point where the edge of the barrel is. When the drill bit gets to the tape, you have gone far enough.
The rifle is equipped with a touch hole liner already. It is probably still pretty clean. I would take it out of the barrel and coat the threads with some sort of anti seize compound to keep it from getting welded in place. Choke lube is good for this. What the touch hole liner is, is the stainless steel bushing that mates up with the flash pan of the lock.
Let us know how this helps the performance of your rifle.
The nice thing about having touch hole liners doing this is that if you do drill too big a hole, the liner is easily replaced. Replacements can be gotten from Lyman, Dixie, Track, etc.
Caveats: The larger the touch hole is, the more it is going to spit when fired. Be especially considerate of people standing to the side of the rifle. Also, the larger the touch hole, the more gas is going to be spit out the side, thus slowing the velocity of the ball. It is not much, but it will make a slight difference. If you are just shooting offhand, it is no big deal. If you are shooting serious bench rest, it might matter.
Wishing you more satisfaction with flinters than I ever had. For the life of me, I just cannot become one with a rock lock. And I have surely tried.
John