The symmetric but slightly too strong gestalt 20 loses 3//4 levels:
1 L
2 LR
3 LR
4 R
5 LR
6 LR
7 L
8 LR
9 LR
10 R
11 LR
12 LR
13 L
14 LR
15 LR
16 R
17 LR
18 LR
19 L
20 LR
or
1 1//0
2 2//1
3 3//2
4 3//3
5 4//4
6 5//5
7 6//5
8 7//6
9 8//7
10 8//8
11 9//9
12 10//10
13 11//10
14 12//11
15 13//12
16 13//13
17 14//14
18 15//15
19 16//15
20 17//16
The symmetric but slightly too weak gestalt 20 loses 5//5 levels:
1 1//1
2 2//1
3 3//2
4 3//3
5 4//4
6 5//4
7 6//5
8 6//6
9 7//7
10 8//7
11 9//8
12 9//9
13 10//10
14 11//10
15 12//11
16 12//12
17 13//13
18 14//13
19 15//14
20 15//15
or:
1 LR
2 L
3 LR
4 R
5 LR
6 L
7 LR
8 R
9 LR
10 L
11 LR
12 R
13 LR
14 L
15 LR
16 R
17 LR
18 L
19 LR
20 R