I would use a small handful. You could use less if you don't have enough. I like to top dress with another handful.

I have used up to 25% castings mixed with bag soil in 12 gallon containers. I have never seen burn. However any container mix greater than 15% castings seemed to restrict/slow water and air movement. My castings are well done and very fine. I now only use in containers as described above. I also use as a seed starter and brew teas. I have enough toI mulch my fruit trees and winter garden beds with it. Most of my castings come from horse manure feedstock and does not burn.

Hope this helps and good luck!