Rolf has recently written a document describing how one should fill out an IO chassis and how the numbering works out. This can be found in the DCC at Rolf's PCIe numbering guide (T1000523).
Basically it works out that slot 1 corresponds to PCIe number 1, but slot 2 corresponds to PCIe number 6. And so forth. The following table gives a quick summary.
| Slot |
1 |
2 |
3 |
4 |
5 |
6 |
7 |
8 |
9 |
10 |
11 |
12 |
13 |
14 |
15 |
16 |
17 |
| PCIe Number |
1 |
6 |
5 |
4 |
9 |
8 |
7 |
3 |
2 |
14 |
13 |
12 |
17 |
16 |
15 |
11 |
10 |
|