If I’m not mistaken, the 600k requirement for RHR is the daas yachid of Rashi, and most eruvin rely on this.
It’s actually far from a daas yochid; many rishonim hold this way. According to the Mishna B’rurah’s calculation, there is a slight majority who hold that 600k is necessary for a r’shus harabbim. Others have a different calculation based on rishonim that the M.B. didn’t see.