Start off at 1/4 dose again..it's different nutes and it's not cause it's bigger that you can't burn it by over feeding it.
The full strength dose on the bottle is just a guide...some plants take more some take less. If yours take less and you give it more, you're screwed
I'd suggest also flushing it before commencing flowering
Off the top of my head white spots are caused by nutrient solution isplashing onto the leaves, spider mites, ph fluctuations..and um I dunno what else if anything. I don't think there's any nutrient deficiences that cause white spots either
Anyways if it's not progressing and your new growth is fine then there's nothing to worry about....just keep an eye on it