Deviation from Default TSS Removal Efficiency

I am working on a project in Germany that is using multiple buffer strips to remove TSS from runoff before it reaches a small drainage channel. Rain water from the roof of the building is collected in a cistern for toilet flushing. Rainwater falling on the asphalt parking (2% slope) area flows (sheet flow) across a 10 ft long grass buffer strip (1% slope) and then across a 70 ft long forested (large pine trees, small other trees, underbrush) buffer strip (3% slope) before reaching a small drainage channel. Because of the dual buffer strip combined with the relatively low slopes and long distance to the drainage channel, I think that the actual TSS removal is higher than the default 65% taken from the EPA’s Guidance Specifying Management Measures for Sources of Nonpoint Pollution in Coastal Waters manual. I think that the TSS removal of the bufferstrip should be closer to the 80-90% range but I do not have a local resource or a good equation to use that would justify this. Is there a way to claim greater TSS removal efficiency than the default 65% removal efficiency for my case that would be acceptable to GBCI reviewers?