I would not add a damn thing, myself. Instead, I change the coolant every 3 years or so, no attention given to miles.

I can't do the math for how much it would take to lower the coolant to -34 and thinking about it, I do NOT think the relationship is all that linear anyway.

I'd let the car sit and get cold, suck out maybe 1-2 quarts and add 100% antifreeze, run the car and then test it again.