When I hear flat salary, I figure they are going to fly me until I drop.
When I hear base plus a day rate, I figure they have no business and better be willing to live on base pay. I put absolutely no faith in the "We generally fly X days per month so your pay will be about Y". If you REALLY fly that often, just PAY me that much.
Why not pay a flat salary and demand they are available X days per month?