What is a more eloquent way to say "I hope I'm not asking too much"?

Something like If you would be so kind, I have one final question of your expertise... should do. Flattery will get you everywhere.

I think it would be good to come right out and thank the person for his/her generosity in answering all of your questions. It might be easier to thank the person, and ask him/her to tell you if/when your continued questioning is taking too much time, or he/she is too busy to answer than to go the "I hope I'm not asking too much" route... this way the person can politely set a boundary if/when it's necessary. The person helping you will likely enjoy the fact that the help is appreciated.

I hope you understand my concern on this very important matter, and I would appreciate any help that you can give me at this time.