"Work out the approximate wing area in square feet (height times half the span). Multiply this figure by 3 for light winds, 5 for normal flying, and at least 8 for breezy conditions - to get line breaking strain in pounds... "
While some experienced flyers manage with one reel-and-line for everything, they are careful to select the right kite for the wind, much as a golfer chooses the right club for a given shot. On the other hand, if you have only one kite, then you will need light line for the light wind end of its wind range, heavy line for the strong wind end, and perhaps an in-between line for in-between winds.
There's really no way around this; light line may simply snap in a strong breeze, and heavy line will weigh the kite down in light winds. If the line starts "singing," then it's time to get the kite down fast, before getting into serious trouble. If the line's too heavy, there will be a pronounced "belly" in the line and the kite won't reach much of an angle.
For example, I am using 50lb and 80lb line for testing new frames on my Wildcard and R6, and I know people who use 100, 110, or 120lb line on their Wildcards - not because they need the strength per-se, but to keep the kite-and-line, as a system, manageable in blowy weather.Best of both worlds: aramid or coramid fiber lines, which are very thin yet exceptionally strong.