Take the single mode OTDR with 1310/1550nm as example, in the actual test, the loss with 1310nm is near to 0.33dB/KM, the loss with 1550nm is near to 0.22dB/KM. We know during the test, the fiber quanlity, connector quality, connecting close or not, etc. all would infect the loss and then final test distance. 

Component loss reference：

Connector：0.2-0.5dB/pcs

Splicing point：The average fusion loss in both directions cannot exceed 0.08dB

Splitter：1/2 around 4dB, 1/4 around 7dB, 1/8 around 10dB, 1/16 around 13dB, 1/32 around 16dB, 1/64 around 19dB

So if you can know more information about your network, then you can make it more precision.

For example, EXFO Max-715B OTDR, 1310/1550nm, 30/28dB. If it is a very good network with one connector and 2 splicing points, the testing distance with 1310nm: (30dB-0.4-0.08x2)/0.33dB/KM=89KM.  The testing distance with 1550nm: (28dB-0.4-0.08x2)/0.22dB/KM=124KM.