I'm going to go with a 'scientific approach' guess.
You're doing 50kg now, you weigh 79kg, have a previous shoulder injury, and have set a 6 week challenge for your self.... I'm to go with 65kg which is 30% increase in weight. Then I'm going to add 3 more kg because others have guessed 65kg, and 68kg represents 86% of your body weight. So 68kg <-> 86% looks like a good number.
As you've gathered by now, my scientific approach is a BS number pulled out from the thin air, but hope you will make it! Good luck!
Thanks for the creative and fun comment😅. But the 50kg I do are for my workout where I do standing overhead presses with 5 sets of 5 reps. For the challenge, I can also use a different method to push the bar up with my legs together with my shoulders. So I could probably do that with 65kg today (although I haven't tried).