What I would do Denny, is not worry so much about hitting 450 RA.
But if you want to really go for it you can -
First, don't add the CaCl2 to the mash, add it to the kettle. That is lowering your RA in the mash and is more important in the finished beer than anywhere else.
Second, don't treat your sparge water. That will dilute the ions you're adding to more acceptable levels in the finished beer.
Third, add as much CaCO3 as you are comfortable with to get the Ca ppm where you are comfortable. Say 150 ppm. That's 1.1 grams per gallon according to the EZ water spreadsheet.
Fourth, add NaHCO3 to get to your target RA. EZ water says that's 2.2 grams per gallon.
Last, Dublin water has a RA of 261 according to the EZ water spreadsheet. London's is 85. Burton's is 262. Dublin and London are historically known for their dark beers, Burton for pale ale. RA isn't everything.
<edit> I can't believe I forgot to say RDWHAHB!