Gary R. Lloyd wrote:

At what outdoor dewpoint temperature do we need to start adding
moisture in order to maintain our 70F and 40% RH?

When 160x0.075(wi-wo) 0.0116, ie wo wi - 0.000966 = 0.005319 and Po
= 29.921/(0.62198/wo+1) = 0.2357 "Hg and Td = 9621/(17.863-ln(Pi))-460
= 40.2 F.

It seems that when outdoor temps start dipping below freezing at night,
our options are to humidify or to accept lower indoor humidity.

I wonder why you wrote that. Another option is air sealing to raise the RH.
BTW, we've only discussed the outdoor dewpoint (vs. dry bulb) temperature.

On the other hand, if our humidity drops below 40% before the outdoor
temps drop below freezing at night, then we need to seal up the house.

Air sealing is good in any case. "Make it tight, and vent it right."

Nick