Salt build up and EC

So I started my first grow and it's in coco. I planted my girl in jiffy disc that I put in 20L pot.
For first days I've been making a feed with RO water and Canna A+B with 0.3 starting, and going up to 0.6 EC. Today (day 12 of seedling, 4th set of leaves showing), after a weekend of no control I made another feed with over 0.8 EC. That's first time I let it flow through the bottom, and the EC of runoff was over 1.1EC.

Does that mean that there is a salt build up there, or did I already wash it down through this watering?

I doubt that you have salt build up after 12 days. 20L pot for a 12 day old seedling is a bit too large - that over 5 gallons - being able to water properly will be a challenge - it will take a long time for the roots to fill the container. That being said - when the roots are well established - you should feed daily to run off. If you do that you will never get salt build up. I have never tested the EC of run off - I guess that's because I have never owned an EC meter. Lol.