To add to the last two posts, the heavy wall pipe you have is making it harder. I have always tested with 12 inch-0.250 wall.
When preparing the branch, the throat will have to have much more of a bevel than the arms. Otherwise it will be hard to get full penetration in that area. The heavy wall pipe will really fight you here. The "arms" of the branch need very little bevel. I still put a land on all of the branch bevel.
When you have an acceptable fit of the branch on the header, trace out the inside of the branch. I then cut about 1/8 inch INSIDE of the line leaving a little of the header inside of the branch. I have never had a problem with an inspector when doing this. This just makes a full penetration of the branch easier.
If possible ask the inspector what he wants to see, how he thinks it should be done. I have found most of the inspectors want you to pass and will be glad to give advice if approached in a friendly way. At the testing facility more than likely there will be old test pieces lying around. Look at them and see what the better looking ones look like.
Once you have a good fitting branch whittled out, I would strongly suggest making a pattern from it. The next one will be much easier if you have a pattern suited to the way you will torch out the branch.
In case someone might not realise it, when you lay the branch on top of the header to mark the header and tack it up, everything is upside down. To make the test after it is tacked up, it will be turned over. In other words the branch and header will look like a T with the header on TOP. If the branch is the right length it can just sit on the ground for the test although I usually raise it up for ease of welding.
One other thing, when tacking up the branch- header, it is helpful to have a small thin wedge to to hold the space at the arms, They can close in sometimes. Also consider where the straps will be cut out and don't tack in that area.