That's similar to what I've been doing, except I use plastic storage bins. I have three: One for soil that's ready to use, one for soil that's been used and one for mixing. I began with a mix of FFHF and
FFOF and then added earthworm castings, rice hulls, coco coir, perlite, etc.
When starting a new grow, I move enough soil for the grow from the first bin into a mixing bin and add amendments. I don't have a specific ratio. I just mix till it looks good. When I mix, I don't add nutrients. I use 5-gallon pots, so I use 5-gallon buckets from Lowes to measure out how much soil I need. I mix slightly more than I need to use for top dressing nutrients.
After a harvest, I put the just used soil in the used soil bin so it can compost and get ready to be mixed and used again. Sometimes I add defoliated leaves and other organic matter.
While composting, I stir it occasionally and check the moisture level.