I am thinking that it would depend on the way the gov decides or the policy during the time you file for SSDI. I just know that if someone files for ssdi and for me it was based like the five years prior to the date of the application itself and then all the FICA I pain in for the time and for my age bracket and things.

I am not sure about being 80% on its own with no pension or wc or other income. I dont think it could be an entire 80%, as I did apply and begin recievng benifits at age 21. Now over about 14-years have gone by and at the time I began recieving ssdi befifits at 21 I had paid in enogh for my age to qualify for total ssdi and no ssi. I really say best to apply the very day you become for sure not able to maintain full tiime employeement for the remainder of life.