For one, selling cows doesn't grow grass. If you don't get the rain you still don't have the grass.
On your point of math, if you have 30 cows and 30 calves. 30x200# eating on pasture is a significant increase. Especially when calves really start to graze a lot more at that size. That's how most people get in trouble over grazing. They underemphasize those couple months when they have big calves grazing with cows on their pasture. It's also the easiest way to add pounds to your calves before they sell if you do it right.
Feeding cows to raise calves is a loser money wise and on your pasture, long term.
That's not taking in to account light calves, breed back issues, etc if you don't maintain their nutrition just right.
I agree with your about probably needing to cut some cows but that is step 2 IMO. Wean early first, then look at cows. Some of that would depend on how long the OP can hold out and how dire the situation is.
No one said they are losing money on the #300, either. A bird in the hand is better than 2 in the bush.
There is probably more than one way to skin this cat but I've had very good luck selling early when things turn dry. I go for #400 because it's a sweet spot on time, price, etc for us but I don't know the OP exact circumstances.