Um, im a huge Canna buff over the last few years and i use 25 ml riz to a 1 liter spray bottle (w/ ro water) for foliage while clones r in domes. I also found it helps to spray the cubes or plugs down a bit with riz solution, right out the door. Personally i never saw a benefit from waiting till i saw roots.
I always see roots a few days quicker then just plain ro, weather inoculated plugs (rappid rooters etc.) or rockwool cubes.
Riz has really low levels of nitrogen and tons of microbes, never found an overdose prob myself. IDK just throwing in some experience, its what works for me and i can definitely guarantee implementing it early isn't going to detriment anything =P