You are correct about the proper location for the vent hole.
There are several things you can do to improve the ignition speed/reliability.
Check the vent hole size by trying to insert the shank of a 1/16 diameter drill into it. If it won't go, then carefully drill the hole out using that drill bit. Go slowly so that the bit removes all of the drilled out metal. You don't want to kick up a big burr on the inside of the barrel.
Remove the material from the bottom of the pan.
There are a couple of things to watch for while your doing this.
First, you don't want the edge of the pan which is against the barrel to be lower than one of the barrel flats.
Second, on some locks the mainsprings upper arm pushes against the bolster right about where the pan is. If this is true with your pistol you don't want to remove so much material that the springs end tab will break thru into the pan.
(As you can tell, I don't own a Trapper pistol and your lock may have a coil mainspring. If so, you can obviously ignore my last comment. :grin: )