For a 3.25lb test curve rod, I use either 3 or 3.5 oz leads, and always the same on any trip.
Basically I work out which one I need to get the distance I require on my furthest cast. At the moment the longest I can cast due to the weed is 50metres, so I'm using 3's.
If I need to cast further, 3.5's cast maximum distance on my rods.
If I'm fishing closer-in with my Specialist rods, maximum lead will be 2oz.