[Support request] Question about EC value with Source Fertilizer and RO water

Hello,
I read whole section on Source Fertilizer site but I cannot find nowhere how to use this range with RO water. In advance guidance its only written this:
EC levels of the water within your reservoir when plants are young should always be set low, ideally 1.0-1.4 EC, if the starting EC of your plain water is around 0.6-0.8 EC.

So my RO water is 0 EC and I guess I’ll get deficiencies later in flower if i dont up some of nutes ? How to do that ? Should i apply Source calmag to my RO water to achieve 0.6 EC and then follow guide from site ?

My tap water is very bad and has over 300ppm calcium bicarbonates so I cannot use it.

Thank you for your help.