Unless you live in
France or a French-speaking area, I don't think you'll have too much of a problem.
Madeline *looks* like 'LINE' and it's complementary to the way you pronounce
Grace CaroLINE.
There's no one correct way to pronounce a name, imo, especially a name that is as universal as Madeleine/Madeline (et al). Pronounce
Madeline the way you like best. If you encounter a different pronunciation, then correct that person.