If your water, before you add nutes, is 625, you probably won't need a drop of calmag. This mostly depends on what's already in your water.
If you can find out at least a general idea of what's already in your water, it will make mixing nutrients a lot easier.
I grew with 650ppm well water for YEARS with absolutely no problems. Until I got a specific strain that just didn't agree with it, then I finally broke down and bought an RO system for 60 bucks off Amazon. Dont waste money on anything more than 3 stage RO system for growing, anything more is overkill.
You can usually find out what's in your water if you search around on Google enough. If you know what aquifers are in your area they will have reports on minerals found in their water, this is usually pretty close to what you are going to get out of your well. Assuming you have well water. At 625ppm I sure hope it's well water