As Mike stated, I would also recommend designing for the Max. Arresting Force (MAF) that is listed on the lanyard. MSA, Miller Fall Protection and Capital Safety are some of the major brands that certify the MAF's and I am sure other do too.
The only difference is that I would use that MAF load unfactored and use a F.S. of 2.0 in the steel design. Using the 13th Ed. ASD, the F.S. is the Omega Factor out front.
Normally, I use the number of users * MAF up to 2 persons and then 300# or so for every users over 2. This is similar to the recommendation of "Introduction to Fall Protection" by Nigel Ellis. This is a great read for anyone designing fall protection systems.
JWB