I have a 5 gallon and a 3 gallon pot. With the 5 i will typically boil my volume at 4 gallons which boils down to 3 so i have to top with 2 gallons. I usually boil my top up water the night before and chill in the fridge or if i'm pressed for time boil and cold water bath during my brew day session. Is boiling the top up necessary or could i just filter with a brita pitcher?