Yeah, water chemistry can take some work to get the grasp on, and there is actually a lot of individual opinion and belief on how to approach it and solve it.
I know you are saying your 54.6ppm is "very soft" water, which I'm sure it is, but get ready for this... straight from my local published water report:
Total Hardness ppm 11.6
So as you can imagine I've been working hard on figuring out how to adjust my nearly empty tap water for brewing. The Bru'n water calculator linked in the prev post is very comprehensive but doesn't do a lot of hand holding so you might want to check out something a little more simple to get the hang of adjustments and target numbers. A good simple one is http://www.ezwatercalculator.com/
The most common additives I've seen in all areas discussing the topic are gypsum, chalk (calcium carbonate), epsom salt, baking soda, and calcium chloride. I was able to get all of these items locally except for the calcium chloride which looks like I'll have to order it online, but based on my results with the calculators, I have a lot of control with the others.
Seeing that theveganbrewer is in Beaverton (probably has extremely similar water to me) and has a lot more experience than me, I'd trust in the completeness of the Bru'n water calculator and its profiles. Might just looks a little overwhelming at first.