They could use the last closing price, but they also could set 1 common = $1.00 before dilution. So you are right.
But psychologically, using $1.00 per common works better if the commons traded at $0.70 before. If they were still at 2.50$, using 1.00$ might be considered offensive ;-)
Most important is that the trading is halted, so they can do whatever they want.
This is the easiest and most correct answer. Applying Occam's Razor, the logic has nothing to do with F&F. I haven't been here a for a while and after checking in, the churn of discussion has a similar tone though the details regarding the law have evolved. The only principle guiding the future of F&F is that of monotonically increasing the size, scope and authority of the government. Ending the conservatorship accomplishes none of that.