Are you talking about the simple difference in dosage or the difference between successive redoses over one experience?
For me I would say as soon as I take a mere 25-35ug or so I can feel LSD's effects, albeit a mere +, and upon reaching 50-60ug I'm edging into a ++ on the Shulgin scale, at 100-120ug~ I'm at a solid and quite visually and mentally strong +++ (albeit a very easily controllable and comfortable one that feels much more easy-going than a typical psychedelic +++ on other substances). Since a ++++ is more of a transcendental type experience that can occur with or without the drug itself, with the right frame of mind, there's no real dose of any substance that can guarantee its occurrence (though the inclination for it to occur does increase somewhat at higher doses).
So dose wise I feel the biggest differences are noticeable up to around 150ug, after that point the trip still gets stronger and stronger, with seemingly no limit until MUCH higher numbers, but I would compare the Shulgin scale ratings to plateaus with DXM and would hasten to say that even the strongest +++ has the same quality as the weakest +++. I would never require more than 150ug for any level of intensity I wanted provided the set and setting were ideal, but if I had lots of extra material to work with and wanted to guarantee and overwhelming number of visuals and mindfuck then I think there's little limit to how high you can push the dose.
As for redosing, redosing LSD is very ineffective when compared with the first dose. If you take 50ug the first dose, and then take 100ug and then 150ug later in the night, it will probably be weaker through the entire night than simply taking 100-150ug to begin with. The first dose is most important, so if you must redose, make sure that you get the first dose high enough, then just use smaller recurring doses to keep you at that level of experience.
