Find the correct gutter fall per metre for proper drainage.
Gutters are hung to a fall, not level, and the fall is specified per length rather than as an angle: a quarter inch per 10 ft is the standard minimum, which is a slope of about 0.2%. What matters is that the fall applies to the run each downspout serves, not the whole gutter. Put a downspout at each end of a 40 ft run and each half only falls 20 ft, so the total drop is half an inch instead of an inch — visually invisible and better draining. That is the trick on long elevations, where a single downspout would need so much fall that the gutter visibly diverges from the fascia. Capacity is separate: a 5 in K-style gutter with a 2 by 3 in downspout handles roughly 600 sq ft of roof, so a large plane needs more outlets regardless of slope.
Gutter Slope
Run per downspout = total run / downspouts; total fall = (run per downspout / 10) x fall per 10 ft; slope% = fall / 120 x 100; downspouts = roof area / 600 sq ft
A quarter inch per 10 ft toward the downspout is the accepted minimum. Half an inch drains faster and is worth it on a short run, but over 40 ft it becomes visible against the fascia line.
Add the downspout. Splitting a long run halves the fall needed and doubles the drainage capacity, whereas steepening the gutter looks wrong and does nothing for capacity.