I wouldn’t worry too much and base the calculations on the current temperature of the beer. If the beer has been warmed up before that and lost CO2 then it will also have reabsorbed the CO2 as it cooled down. One issue here is that as it absorbs CO2 from the headspace it would pull air into the airlock and thus lower the CO2 pressure in the head space. This will lead to less residual carbonation.
That is the part about Henry's Law that I'm not sure about. Since my chest freezer is holding CO2 in, let's assume that when the beer cools back down and when it sucks gas back through the airlock that it is all CO2 coming back in. The part I was wondering about is after the beer warms up and loses CO2, when it cools back to its original temperature does it pick up the same amount of CO2 that it lost, or is it a lower amount because the beer was originally sitting in a supersaturated state? Since the CO2 partial pressure is the same before and after in this example, I suppose Henry's Law says that it would eventually take up the same amount of CO2.
I guess another way of looking at it is, say I have two beers that have just finished fermenting. One I leave undisturbed and the other I use a wine whip and degas as much CO2 out of it that I can. If I let them both sit under the same amount of CO2 headspace pressure, do they eventually end up with the same amount of dissolved CO2, or does the undisturbed one hold more because it started from a supersaturated state?