Nice. It looks like they included July 2016 to derive the 286 average but it is clearly not representing the changed law at only 79 vehicles. The numbers only really jump from August 2016. If you take the past 13 months back to then (excluding July 2016), the average is 301. There seems to be no exponential increase in the past year - very linear. So, excluding Model 3's, I think a 300/month projection is a good number to use - which takes us to early April as you suggested, with the tax exemption ending at end of May.
Hard to estimate how many model 3's would be sold in Washington before then - I guess there is a breakdown of tesla sales by state somewhere?? Though at the rate that M3 production is going, I don't think many people here are going to get a M3 before the tax exemption expires ...