Light Loss Factor or also known as LLF in short form is one of the hardest parts to remember in the lighting industry. Each fixture can have a different value and some are better designed to eliminate some of the common losses we encounter when doing a lighting design. Each manufaturer of fixture, or lamp would have it's own value to be totally accurate. The IES files provided to not include such light reductions so experience in this field is mandatory.
For example, a fluorescent fixture needs to be roughly around 0.7 to 0.8 LLF total which should include reductions due to ambient room temperature and ballast factor if it was not done already on an IES file. Some parts may be already done in the IES file so that makes having tools to read these files extremely important. LED's have come a long way and have far less light loss. Dirt and Dust play a part still as they'll always interfere with light output. We typically do not measure a design with dust and dirt in it so we leave that out. For LED we typically reduce the LLF to .95 or by 5% to simply add a little fudge factor to a layout.
It's always better to have a little more light in a layout to be safe.