Can someone explain me how to determine the % on a straight draw?

[Shandrax]

Let us assume hole-cards + flop give me 5-6-7. What are my chances to complete the straight on the river?

This is what I have tried so far:
4/47 = 8,5%

now I can hit the upper or the lower end of my straight:

8,5% x 2 = 17%

then I have to hit the remaining card:

4/46 = 8,7%

then I multiply 17% x 8,7% = 1,48%

in the end I double that because I have two ways to hit my cards. Like 9-8 and 8-9 for example.

1,48% x 2 = 2,96%

The correct number should be 4%, so where is the mistake?