I was told that "most people" only need 10-20 mg when taken for public speaking. I was prescribed propranolol for public speaking, which is a bit different than what you are using it for. However, I would experience blushing, trembling voice and limbs, etc.
I figured out that I just need more. I never felt any relief unless I took 40 mg. I usually take 60 mg when needed, but have taken 80 mg.
It works great - I just needed a higher dose. I am female, 5 foot, 5 inches, and 130 lbs. The amount you need also depends on your size. At my size, I experienced no serious adverse effects even on 80 mg, except for fatigue when the propranolol wore off.
Ask your doctor, of course, but you should be fine taking a higher dose. Propranolol worked so well that I now volunteer to speak at events. I was a panelist recently and spoke in front of 40 people. I'm not as scared because I know propranolol won't fail me.
I take it 1 1/2 hours beforehand. It should work within 1-2 hours.