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Your goal should be to try and get the receive levels on your optics within the thresholds given by the optic/manufacturer. This applies to longer distance DWDM and CWDM optics (40k, 80k, 120k).
Always pad optics on the the rx ports (one pad on the rx port of the hub interface, one pad on the rx port of the cpe interface).

Notes/Key:

Standard estimates:
- 0.4 dB loss per kilometer for 1550nm
- 0.3 dB loss per connector
Command line to see reported RX high and low levels and current levels:
- Juniper:
  show interfaces diagnostics optics xe-0/0/1
- Cisco:
  show interfaces gigabitEthernet 0/1 transceiver detail
- Brocade:
  show optic thresholds 1/1/1
The optic vendor should also have the rx and tx thresholds documented:
- Example: https://www.flexoptix.net/en/sfp-plus-zr-transceiver-10-gigabit-stm64-sm-1550nm-80km-23db-ddm-dom.html
  - RECEIVE MIN. -23.00dBm
    RECEIVE MAX. (RECEIVER OVERLOAD) -8.00dBm
    TRANSMIT MIN. 0.00dBm
    TRANSMIT MAX. 5.00dBm

Calculating Loss:

Record the TX power of your optic
Subtract the OTDR results (loss in dB)
1. If you don't have OTDR test results, subtract the estimated loss based on distance
Subtract the connector loss
Record the result
Is the result within your optic's receive min -> max? If not, add the appropriate pad

Example:

I'm transmitting at +1dB
I don't have an OTDR, but the CAN says the distance is 27km
1. Subtract 0.4 dB loss per kilometer for 1550nm at 27km, subtract 10.8dB
I'm using two patch cables, one on each side, subtract ~1dB
Result: −10.8
I'm within the -8dBm to -23dBm receive specification for the optic, I probably won't need to ship a pad