Basic Nutrient Strength Question

[JHazzardB]

Hey guys! First post and super excited to finally join the forums.

I have a basic nutrient strength question. I use tap water for my reservoirs (not the best) with a PPM of 285 and EC of .58 or so. It can vary daily or weekly by a smidgen.

If I'm aiming for a nutrient strength of say 900ppm (around 1.8-1.9EC) to feed some tomatoes, I should be looking for a final reservoir PPM of 900+285=1185 correct?

I use the Advanced Hydroponic nutrients with the 3 part Micro, Grow, and Bloom (pH perfect) stuff.

Also, I haven't learned the way to make my own nutes yet or follow any custom recipes. However, if I know what is in the Micro, Grow, and Bloom nutrient bottles, can I tell how many PPM of N-P-K and Ca I'm adding? I also have a bottle of General Hydroponics CalMag (1-0-0 5%Ca and .1%Fe)

For instance:
Micro bottle is labeled as 2-0-0 with 2.4%Ca and .05% Fe
Grow bottle is labeled as 1-0-4
Bloom bottle is labeled as 1-3-4
Mixing in equal parts is 4-3-8 with 2.4% Ca and .05% Fe

From knowing this, can I tell how many PPM of each nutrient I add when I raise my reservoir by the desired 900ppm?

I'm not good with numbers and have started rambling with excitement. I don't have any strong feelings any way to try to hit the 900ppm or 1.8EC target, I'm just trying to get a good understanding. Thanks in advance everyone!