It is impossible to have total alkalinity go below zero by boiling water hard/carbonaceous or not. If one adds acid to the water until the pH reaches 4.3, then 50 times the number of milliequivalents of acid required per liter of water is the alkalinity. Thus for your water with alkalinity 273 it would require 253/50 = 5.06 mEq acid per liter. Negative alkalinity is achieved when one continues to add acid at this point. For -193 you would add 193/50 = 3.86 mEq additional (per liter).
What actually happens when hard carbonaceous water is boiled is pretty much anyone's guess. If you plan to use this technique as a matter of course I would definitely invest in a hardness test kit of the type that gives calcium and total or magnesium and total hardness and an alkalinity test kit (www.hach.com
) so that you will know what really happens. A rule of thumb is that alkalinity or hardness, whichever is limiting, can be taken down to about 1 mEq/L (50 ppm as CaCO3). Assuming this to be the case and taking calcium down to about 1 mEq/L would drop about half the alkalinity so you would have alkalinity of about 130, calcium at about 19.3 mg/L and Mg at about 28. RA would be 100 down from 212. If diluted 1 + 1 with RO water alkalinity drops to 65, calcium to 9.6, magnesium to 14 and RA to 50.
Are you perhaps confusing total alkalinity (TA) and residual alkalinity (RA)?