Well there's two ways you can go here.......actually 3 I can think of. One would be to just use the water at 300 and skip the CalMag, second would be use the britta and the Cal Mag.

What exact number did your tds number show you in your res? It sounds as though your meter reads x 500 conversion......mine use x700 so your numbers shouldn't match mine anway.

There is no one sweet spot.....as your plants grow and change so will their needs. The only time I've ever kept the number the same is in later bloom when they are at max performance. Hydro really isn't made for the "set it and forget it" crowd although there are those that do it that way and can make it work. I'm just not one of them.

You are low on options.....the size of your rez; your tap water above 300......option three would be to change your e and f table to something like DWC or drip...another type of system that wouldn't require a 40 gal rez and THEN buy an RO machine. This is simply something to think about for the future.

Also with your ppms......start low and raise the number every few days; keeping a VERY CLOSE eye out for the signs of overnute that tell you to go no higher. You can read the symptoms for overnute in my Feeding Schedule.