Several have responded to similar questions. If you click on the search drop down arrow, select Questions, then type in rusle LS factor. It will bring up many questions of the past. If you see some similar or interesting, select Answers, and you can see the past answers to those questions. I think a substantial amount of information is available with this technique.
In past answers, it is obvious that there are different ideas on how to do this or to estimate in GIS.