For container growing, I strongly prefer chemical fertilizers in the 3-1-2 ratio as they're readily available with micronutrients, are closest to the proportions of major nutrients that plants ACTUALLY use, and roughly represent the nutrient percentages you'll find in tissue analysis (which is the basis of fertigation practices adhered to by commercial operations). When using organic fertilizers in container culture, delivery can be unreliable, slow, and/or erratic. You may not get the nutritional boost you need for a long time after you apply the fertilizer because it has to pass through the gut of microorganisms before it's broken down into usable elemental forms. Also, in highly organic soils like you're probably using, it tends to carry over between applications and accumulate.

Since microorganism populations are normally boom/bust in containers, you can go from no nutrients being released to too much being released in a very short time. Elements can technically be abundant in the soil, but unavailable until complex hydrocarbon chains are cleaved, so even with plenty of N or other elements potentially available, nutritional deficiencies are entirely possible and they can swing to toxicity levels simply by way of an increase in soil biotic activity.

Some of the things that affect the microorganism populations and availability of nutrients are moisture levels, pH, level of accumulated salts, and soil temperature. Any and all of these things can, and do cause sometimes wild fluctuations in microbial populations and therefore nutrient availability.

It's hard to say how much fertilizer is appropriate. I'm thinking that 1-1/2 cups of fertilizer on 10 sq ft of garden seems like a lot of fertilizer & I doubt there is a way you could make any kind of guess at a conversion to cu ft in a container w/o knowing the source of the nutrients, especially N. For example, if the source of N is bloodmeal, the amount of fertilizer you could apply would be far less than if it was feather meal.

I can say that the 4-6-3 blend you are using has about 6-7 times the amount of P that you need for lettuce.

Al