There's probably a better way to do this but I think this will generate what you want:

    nsplit = function(X,n){
     p = X/sum(X)
     diff(round(n*cumsum(c(0,p))))
    }

where `X` is a vector of sizes and `n` in the total. Example:

    > nsplit(c(1,2,3,4,5),123)
    [1]  8 17 24 33 41

Those numbers sum to 123 and are approximately in the ratio 1:2:3:4:5.  See also this:

    > nsplit(c(1,2,3,4,5),10*sum(1:5))
    [1] 10 20 30 40 50

Note if any of the ratios are small or `n` is small then you will get zeroes:

    > nsplit(c(1,2,3,4,50),20)
    [1]  0  1  1  1 17