Initial extent is the number of bytes your object preallocates when it's first created. Next extent is the size of all following initial extent extents. Maxextents is the maximum number of extents you're allowing per object. It could be a set number or unlimited.
The goal is to keep a fewer extents in any object.
I know it is a good practice not to have more than 2 extents for an index. If your index has more than 2 extents you can rebuild it with right parameters.
If you know or can estimate the size of the data in the table make your initial extent equal equal to the number of bytes you're predicting to have. If you don't know what you're going to have make your best estimate and try to keep as little extents as possible.
To find out how many extents are in the object you may run the following query:
select extents
from sys.dba_segments
where segment_name like '%<your object name>%'
and segment_type='<your object type>';
As to pct increase, I think it is better to keep it 0.
Hope it brought some light on your question. This would not be a complete answer. May be someone can add more.