I believe the default precision and scale changes as you change the scale.

For instance if you set precision and scale for casting as (4,0), then spark will default it to (10,0).

For (8,1), it'll default to (11,1)

For (8,2), it'll default to (12,2)

For (8,3), it'll default to (13,3)

For (8,4), it'll default to (14,4)

For (8,5), it'll default to (15,5)

For (8,6), it'll default to (16,6) and so on..

I think the logic is:

if precision-scale < 10:
    precision = 10+scale
else
    precision=precision