Should I do reflection before eating if I don't plan to keep it Should I do reflection before eating if I don't plan to keep it?  

There is an issue I'm aware of that you shouldn't say stuff you won't be able to do  - I mean sometimes you can but that its better not to promise stuff you probably will fail doing (I don't want to give examples but it is a thing in Buddhism I hope some will know what I'm talking about) 

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 isn't going to keep it? What about when he thinks there's a tiny chance he will?  What about most likely will 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.