I have tried it many different ways. If I am chording, then putting it flat makes most sense. Usually I am playing it with the flexible tube holding it in my left hand about 18″ 2 ft to my my face, angled toward my face and down. That way I can see the keyboard. I have been playing with a choro ensemble, which combines pretty fleet single lines and chording. And I just saw Hermeto Pascoal, who had an amazing melodica player with him (well amazing pianist who also played melodica) Jovino Santos-Neto. He just used the mouthpiece, as does jon baptiste. The length of the keys is not the issue for me, its the width.
Thanks for your help/ . Eric