Hello miladg,
from my point of view if you attempt to use the 0/100 approach, you must model the input material as 100% virgin material and include it into the system boundary.
This is because it is assumed that in the same way as you claim the credit for your recycled output, somebody else has alredy claimed the credit for providing the recycled material that you are using. Therefore, you are not allowed to claim the benefit of using the recycled material in your product. This seems counterintuitive at first, but it is the nature of the approach.
I recommend reading the chapter on the CFF in the PEF method.
https://eur-lex.europa.eu/legal-content/EN/TXT/?uri=CELEX%3A32021H2279
In the formula, if the A-factor is 0, the formula represents the 0/100 approach. As you can see, in that case the recycled input (R1) also carries the burden of virgin material (Ev)
Hope this helps,
Paul