I am not sure how you would extrapolate one way shear forces from a finite element program. Hand calculations are better suited to calculate one way shear. You should use the maximum shear, not the average shear force. How thick is your raft?

I am talking in general case.
The finite element programs gives shear force per meter length in a specific vertical plane.
When I choose a vertical plane parallel to a wall for example, I get shear forces per meter length on each node of the plane. If I sum the forces over the whole plane I would have the shear V in T similarly to that of a beam.
The thing is that I get areas on the planes where there is little shear force(T/m) resisted by concrete strength so I don't put reinforcement for them. Plus, taking the whole shear force and apply it to the whole width will give me much lesser reinforcement, by that much lesser reinforcement in area where the peak shear force per meter is(where the failure starts to happen). That's why I divide the plane to 1 m width segments and for each I calculate the required shear reinforcement, by using the average shear value x the width of the segment, or to be more conservative, the maximum shear value x width of the segment.
I want to be sure if this is correct. If not how should I check one way shear in slabs and rafts? and how to provide reinforcement?

The best discussion of this that I know of can be found here: Link

As you've discovered, the trick is determining what an acceptable integration width is for one way shear design. You method sounds reasonable. The method in the link might shave off some effort if you choose to adopt it.

I like to debate structural engineering theory -- a lot. If I challenge you on something, know that I'm doing so because I respect your opinion enough to either change it or adopt it.

Is this method in p 15(". Recent practice has adopted the use of an effective width equal to the width of the superstructure element imposing the demand, plus one mat thickness either side of the
same element. ")applicable for slabs also?

Great question. To my knowledge nobody is applying that method to slabs. Rationally, however, it would seem that what's good for the goose is good for the gander so to speak. I mostly hang my hat on two way shear capacity in slabs and rafts and tend not to sweat the one way so much.

I like to debate structural engineering theory -- a lot. If I challenge you on something, know that I'm doing so because I respect your opinion enough to either change it or adopt it.