Think about it this way. If you have a sud, and you buy a set of 4 springs to lower it (say) 20 mm then you will get some sort of lowering effect maybe close to 20mm, and it should all work. If you have a 33 and do the same, you should also expect a good result. But if you have to buy different springs front and rear, not designed as or sold as a set, then it may not work out. Hence the trial and error.
What I don't get about lowering springs - when they quote a dimension (like 20mm or whatever), what's the starting point? An early 1200 single carb sud will sit much higher than your late sprint. So lowered 20mm from what?
And to a similar point, do you want standard 33 springs in the front from a 1200 single carb 33, or a 16v qv? They will be very different.