For calcium, you should only need to be concerned with being above ~40ppm and (ideally) not much above ~120ppm. Anywhere in-between should be fine. In the case of matching the "Pale Ale" profile in Bru'N water, you should be focusing on the sulfate and chloride levels, but magnesium and sodium might be worth watching to a lesser degree. From this perspective, you should be fine with the exception that if you have a high boiloff rate then those levels may "concentrate" in the finished beer, in which case you'd target slightly lower values to account for the "concentration" that occurs.
From my own personal perspective, I find 300ppm to be borderline off-putting (IPA or not). I find it excessively drying and tongue-coating. The level of mineralization from 300ppm of sulfate is plainly obvious in the finished beer. In a sulfate-favorable beer like an IPA, I try to shoot for an upper value of about 200ppm (which is about my upper threshold for sulfate levels). This is all just my opinion of course.