I do this for that exact reason. But I use 11 for the 1 day WS, 12 for the 2 day WS, etc…. That way, if I want to preempt the proffer with a specific slip, spot 1 is still open.

to the OP of the question, to keep a WS a proffer, you need to get inside manual trip coverage, which begins after the 0700 PCS run 2-days prior. Remember you need to use a separate slip for each length of trip, because to fully capture all options for a proffered 1 day you need to put flying on the 15-17th, but a 3-day needs to be 15–19 (assuming today is the 15th). Also so long as you remember to say “NO” to the “award on 0700 PCS run” question, you could have input your slip after the 2200 PCS run last night, and before and the 0700 PCS run this morning. (For example inout at 0600 over coffee, but say NO)