You’re doing fine @afre, indeed. First step is always to make something work as expected 
Then, it is just cosmetic.
Here is my proposal, from the version of @garagecoder, except I’m not using an image to store the log numbers, but a single variable s. There is almost no differences, I suspect @garagecoder’s version is even slightly faster by the way
deets2 :
r={[im,iM]} split_details 5
l[^0]
s={"s = vectorl(); for (k = 0, k<l, ++k, s[k] = log((ia#k - im#k)/sqrt(iv#k))); s/min(s)"}
repeat $! *[$>] {arg(1+$>,$s)} done
endl
+ n $r