The answer depends on a lot of thing but ...

If we assume WHZ is normally distributed with SD = 1 then we expect 10% GAM when the mean WHZ is about -0.72. We can see this in R:

The proportion with z < -1 can be calculated:

The difference:

estimates what you are asking for.

I hope this helps.

Dear Haitham Shukfa:
Mark has supplied an excellent answer to your question; however, I would question the existence of this category of "at risk to be malnourished". Children with z-scores between -2 and -1 are normal and should not be labeled as "mildly malnourished", "at risk of malnutrition", or given any other label which might imply that they need any targeted nutrition intervention. Although the old Gomez and Waterlow classifications had a "mild" category, the currently-used definitions of malnutrition using z-scores do not include this category. In a population with normal nutritional status, such as the WHO Growth Standard, statistical theory says that 67% of children fall between -1 and +1 standard deviations and that 95% of children fall between -2 and +2 standard deviations. Subtraction tells us that 14% of children in well-nourished populations are between -2 and -1 standard deviations. Unfortunately, this group of children has been labeled as something other than normal in various reports which then cause panic when children with z-scores between -2 and -1 are included in overall estimates of the prevalence of malnutrition. For these reasons, I would strongly recommend that children with z-scores between -2 and -1 not be labeled differently from children with z-scores between -1 and +2. They are normal and merely represent one end of the normal range.

Thank you dear Mark.
Dear Bradley, you are totally right, but the idea behind this classification (threatened to be malnourished) is that we have a plan to distribute food baskets in a besieged area (rural Damascus - Syria) and we will cover all GAM cases' families for 6 months, but we still have extra amount of baskets, so we reactivate this old criteria (threatened to be malnourished) , and we will cover their families for 2 months.
Make sense!!