How to calculate whether or not to use attenuators

Can anyone help me get some rule of thumbs to calculate whether or not I need to put in an attenuator and how much dB it should be ?

I know I need to get the output power of the Tx of the SFP, the minimum and maximum power on the Rx SFP, the dampening in dB of each fibre cable (certified measure report which should be included with each patch cable) and how dB I loose over the long link, but I need some rule of thumbs here. The thing is that we're going to use LW SFPs to span only 400 meters (1200 feet) or so, so the output power of a 10km SFP is probably a bit too high and I don't want my SFPs to burn to a crisp in a week or so.

So I need rules of thumb for the dampening of:

- 9/125 patch cable

- patchpanel connector

- 9/125 ground cable per kilometer (or feet, whatever you prefer)

Can anyone help me get the right numbers ? It's been too long since I had to calculate it myself and I can't find any safe rules of thumbs.