When calculating EC based on mfg recommendations, do I also calculate my additives?

I'm using louder powder and it's recommended to use an EC of 2.2mS (2200µS).

My tap water comes out at 250µS.

I add cal mag which also adds a little.

Should I be going for an EC of 2200µS+250µS+calmag so around 2500µS (2.5mS)? Or should I be looking at the final EC as 2200µS regardless of starting points?

The final or total ec would be 2.2ec then

Are you in coco or dwc or? Id say without additives. The chart just gives a idea where ec should be. Even then I found my plants and enviorment etc will determine what ec the plants need. Like now Im finding like others say that their plants grow just fine regardless of strain but others say otherwise. Sometimes strain might matter.

I used to do 1.1 ec in 3 gallon fabric pots coco drain to waste. Then I went to half gallon to a gallon and now they need 1.7 ec. .5 ec is tap water. Also with all the new airflow need to feed 8x a day vs 5x.

So I dont follow charts they wont suddenly fix your plants they either do ok or meed adjustment or total change. Im not very experienced but just my thoughts few years growing.