They are correct. For example, say you had these four measurements:
20.5, 20.7, 20.2, 20.6
You calculate the mean: mean = (20.5+20.7+20.2+20.6)/4 = 20.5.
Then you take half the range of the results to be your absolute uncertainty: (20.7-20.2)/2 = 0.25 (you normally only quote this to 1 s.f.)
Therefore your answer will be: 20.5 +/- 0.3 units.
When quoting your answer the number of significant figures you quote your answer to is very important.
The uncertainty is always given to 1 s.f. In this case that happens to be to 1 d.p., so you quote your answer to 1 d.p.
To calculate percentage uncertainty it's : ((absolute uncertainty)/(mean)) * 100.
Note that when calculating anything, never round until the very end. So to calculate the percentage uncertainty you should use 0.25 rather than the rounded 0.3.
(0.25/20.5)*100 = 1.22% (rounded to 3 s.f.)