Should I do reflection before eating if I don't plan to keep it
So my question is should one say a reflection on food ("i use this almsfood not for beautification but for supporting the holy life etc etc ...") when he knows he probably isnt going to keep it ?


As a lay person: Probably not..

As a monastic: Definitely not!

Reflecting on Alms food belongs to the The Four Requisites, namely:


  "Properly considering almsfood, I use it: not playfully, nor for intoxication, nor for putting on weight, nor for beautification; but simply for the survival and continuance of this body, for ending its afflictions, for the support of the chaste life, (thinking) I will destroy old feelings (of hunger) and not create new feelings (from overeating). Thus I will maintain myself, be blameless, and live in comfort."


As explained in the footnotes/commentary - then a monk should make use of offerings properly and not do anything with them which will make the donors faith decline. 


  What about when he thinks theres a tiny chance he will ?


If his intentions are wholesome, then yes of course.
Even a small amount of wholesome intention, serves to strengthen the Adhiṭṭhāna (resolution) pāramī.


  What about most likley will keep it ?


Yes of course.

Actions that reduce distance to Nibbana should be undertaken.