Distributions from a Roth 401(k) do not follow the same ordering rules as distributions from Roth IRAs. A distribution from a Roth 401(k) is a proportionate mix of basis and earnings, so if you took out less than the entire balance in the Roth 401(k), some of your contribution basis remained. Your contribution basis in the Roth 401(k) was reduced only my the amount shown in box 5 of the Form 1099-R.
No, you do not split the code 1B From 1099-R. It must be entered whole.
