preboil

HI everyone, If I was to boil my water before adding to the mash to reduce alkalinity, how long would I have to boil for? The amount of CaCo3 in my water is 335.5 and I'd prefer not to use CRS as according to Brewers Friend this pumps up my Chloride too high. Any advice very welcomed thanks.

I am fairly new to brewing and have not managed to dial in my efficiency, boil off volume, grain absorbtion etc. and therefore there is a level of uncertainty when it comes to my post-boil volume and OG. This is the plan that will allow me to avoid measuring the volume of my wort after boiling...