Dead heading heleniums

[Paulines7]

I always know when to dead head most of my plants such as pinks and roses as the flower head dies completely, but I am a bit puzzled as to when to take the heads off heleniums. 

It is the first time I have grown them and they have not disappointed me at all.  They look just like they did in Hillier Gardens last year and I am so pleased I bought some.  I am even going to extend my herbaceous border next year so I can grow even more varieties of them!

Mine seem to be in various stages of producing a seed head.  I have Googled them and read that dead heading will produce new flowers, but no site tells me when to snip the old flower head off.  Do I wait until the petals have fallen off the head or should I do it when the petals are drooping? 

Any advice would be much appreciated please.

[Paulines7]