Ok, I think that is too much photo flo. I use 0.5 ml/100 ml of emulsion.
But, most importantly, using that sulfur sensitization I described previously will probably boost your speed to over 25. I've gotten 40 in most cases. You will also get higher contrast.
If you use the prehardener, it is a good idea to wash afterwards and then fully process the film. If the film is left in contact with the formalin overnight, it can fog the film. Formalin can do that. It depends.