Contributing 100% of salary to SD 401k as ROTH

[Shera Gregory]

I have a SD 401k which has an LLC as the plan sponsor. I take distributions from the LLC as the single member. I want to start putting 100% of my distribution into the 401k as a ROTH contribution. If my normal distribution would be, for example, $1,000 can I put the full $1,000 into the 401k or do I need to pay estimated federal income tax out of that $1,000 and only contribute the difference? I think I can pay the self-employment taxes separately since that's not normally withheld by the LLC anyway.