First and foremost, make sure you are reading the bottle correctly. Are you sure you are looking at a 20% concentration and not a 20° Baume? Because 20° Baume IS 31.45%
Once confirmed, the factor is very simple:
Use 31.45% as you baseline.
Then divide it by the %'age you can get your hands on (in this case 20%):
31.45 / 20 = 1.5725
That is your "factor"
Now if you have already determined that you need say 16 oz of the 31.45% stuff to achieve your pH goal (this is just an example), then you Multiply 16 times your factor above to determine how much of the 20% stuff to put in:
16 X 1.5725 = 25.16 Ozs
In this scenario you would need to add 25.16ozs of the 20% stuff to achive the same goal as adding 16 ozs of the 31.45% stuff.
We can logically say this makes sense as you clearly need MORE of the lower concentration to achieve the same with the higher concentration. In this case, 25.16 is indeed greater than 16. So it makes sense.
How much more?
25.16 is X% of 16
25.16 = x * 16
<do algebra and divide both side by 16 to get x alone>
25.16/16 = x
1.5725 = x
Bingo!
But please please please make sure you are really dealing with a 20% concentration Muriatic Acid. because I have never seen that.