The 5-1-1 is more moisture retentive than the gritty mix, which makes extensive use of calcined clay, so I am not sure you really need to add. If anything, you can bump up the peat/coir component of the 5-1-1 if you wish, although I have never found that necessary. I grow virtually all my container plantings in a 5-1-1 mix.

My other suggestion is to water thoroughly before fertilizing. I generally water well one day then fertilize the next. I dislike self-watering or sub-irrigated containers but many container gardeners that use them will collect the extra, fertilizer-enriched liquid and recycle it with other plantings.