About 15° is a good angle to start with, pushing the protective gas just forward of your weld so you are welding into it as you move along. The gas shield protects the molten puddle from the impurities of our atmosphere as you are welding.
Best practice to get the angle down is to take two 12" long x 1" wide pieces of new flat stock gapped at 1/16" laying side by side. Then practice starting at 15° moving the angle until you find your sweet spot for a nice looking, good penetrating weld.
Can test the stength of your weld by putting it in a vice and trying to fold it over. Can also visually see the penetration depth on the back side too.
