PCTINCREASE doesn't really have anything to do with tablespaces extending.

PCTINCREASE deals with the segments inside the tablespace. For example, if you have a table that has a next extent of 10K and PCTINCREASE is set to 50, the next-next extent would be 15K. The next-next-next extent would be 22.5K, etc.

If your PCTINCREASE is set to 0, all extents would be 10K, thus uniform extent sizes.

I like to set the default PCTINCREASE at 1 for the tablespace and then all the objects have a pctincrease of 0. This way, I can tell when my developers sneak a table in without sizing it correctly and don't have to worry about the table extending out really quickly.