Kaye, thank you so much for getting me straightened out on this!
Just as an aside–another wrinkle on this:
I have always used the “before and after” method of allocating the state income tax, because it yields the best results with the steeply progressive state income tax. I.e. tax on all income minus tax on just non-investment income = tax deducted. However, that calculation looks impossible now that I can use only a portion of the tax actually incurred on 2018 income. But I can use the method described in the 8960 instructions. The end result is the same.