If you choose 1.5 main line, then you should choose 1.2 or 1.0 subline.
If you choose a 0.8 size subline, then you should choose a 1.0 or 1.2 size mainline.
Normal carp fishing with a softer rod can choose 0.8 main line with 0.6 subline, which can cope with fish of up to 3 pounds.
If you use a harder rod, you can choose a 1.0 main line with a 0.8 subline, which can also handle up to 3 pounds of fish.