10 ppm from a RO unit is fine and is typical. As pointed out above, the inclusion of a de-ionizing stage (DI) will take out that remaining ionic content. For brewing, the DI stage is unneeded and unnecessary since you will typically want that minor mineral content in the brewing water anyway. For most ales, bringing the calcium content back up to 50 ppm should be helpful. However, lager yeasts may not need or tolerate much calcium, so it may not be necessary to boost the calcium level to 50 ppm for that yeast.