Here's a stepped projection, using decreasing growth rates by decade:

Step one - assume that for the next 10 years, the percentage increase will be 8%: over 10 years that translates to a multiplier of 2.15. That 8% number is actually lower than the number between 1991 and 2011 by the way.

Then decrease that percentage rate by 0.75 point every 10 years until 2080, 63 years from now

Over 63 years here are the multipliers:
2.15,2.01, 1.88, 1.79, 1.66, 1.55, 1.11  - the last one representing a 3.25% increase over only 3  years.

Multiply them all together and you get 41.5: Multiply that by 1.2 million and those numbers project out to a Muslim population of 49.8 million.

Use your own numbers, go ahead. .

If you use a 1 point decline every 10 years, the multiplier is approximately 26, so that would project a Muslim population of 31.2 million.

Go ahead and argue about the percentage growth. But do not argue the math.