Typically:

Line length is a function of kite wingspan. Larger kite, longer line length.

Let's assume you have a 7 foot wingspan kite.

Trick line = 10 wingpans = 70 feet

All-around line = 12 wingspans = 84 feet

Figure flying = 15 wingspans = 105 feet

These are guidelines.

40 feet is very short for a dual line set unless you're flying a smaller kite (4D for example)

Good luck.

I'll go with what John recommends, since I couldn't find my slide rule to check his mathLine length is a function of kite wingspan. Larger kite, longer line length.

Let's assume you have a 7 foot wingspan kite.

Trick line = 10 wingpans = 70 feet

All-around line = 12 wingspans = 84 feet

Figure flying = 15 wingspans = 105 feet

These are guidelines.

40 feet is very short for a dual line set unless you're flying a smaller kite (4D for example)

Good luck.