Tumbling is a star within the Constellation The Shadow in Elder Scrolls Online.
With each point invested, the reduction in the " Stamina cost of Roll Dodge and Break Free" diminishes. That is to say, for the first 10 CP invested, you would gain a 4.78% reduction in Cost. However, the next 10 CP spent (making a total of 20 CP spent), will earn you only 4.26% more reduction. This is 0.52% less than the reduction gained in the first 10 CP.
As such, spending just 33 CP into Tumbling will already gain you a 13.47% reduction in " Stamina cost of Roll Dodge and Break Free". This means you will have gained 50% of the Maximum reduction possible (which is 25.0%) by spending just 33 out of the total of 100 CP. Therefore, beyond this point, the reduction in Cost received per 1 CP used is very small and not as worthwhile.