What it is, if you didn't know, is a mix of Phosphorus and Potassium which are key elements in flowering. You can use that canna product, or other 'budding' additives any time starting from when the first flowers are forming to when you are going to flush or harvest. I think you will have a hard time finding a consensus on exactly when is the right or wrong time to start boosting on top of your general flowering nutrients.
*edit* Actually one more thing, you say 32 days in but each strain is different and sucks different amounts of nutes- so that makes a one size fits all rule even less likely to be found