I haven't had to go through this exact process with EASA, but it sounds like they just want a clearer picture of the risk based on your history. If your allergist is unsure about how to calculate the percentage, they could look at how long you've been symptom-free and if you've had any recent reactions or close calls. When I went through something similar, my doctor used a risk factor based on how well I'd been managing the allergy and how likely a reaction was based on my past history. I'd suggest asking your allergist if they can pull data from your medical history to estimate the risk more precisely.