There will always be the days that come where you may need or want to work on .  or have a few first cleans to do , so 350 may not be big enough depending on your needs .  I carry some barrels as back up and a pump to transfer the barrels into my tank while I work. 
I am also fortunate that I live in a soft water area ... around 50 ppm , so when I need to , I take a DI  with me and top up while I work