Adding EWC

In an organic soil grow what is a good ratio to add EWC at?
I'll be growing autos most likely in Roots Organic soil with Mephisto Super Soil additive but wanted a little extra in the soil. I don't want to burn them of course but aren't EWC's fairly mellow in that regard?

I wouldn't run anymore than 20% ewc is very good nutritionally but it has a tendency to compact after watering over time. This can be counteract ed somewhat by adding a good amount of an aggregate substance such as perlite at 30% of your mix.