image's height in -geometry ?..

Hello.

I would like to put a logo over image:

convert image.jpg logo.png -gravity South \
        -geometry +0+$[height/4] \
        -compose src-over -composite result.jpg

My problem is i don't know how to put image's height into -geometry's
argument (it does not accept percents, does it?).

I could ask "identify":

  height=`identify $im | sed -e 's/.* [0-9]*x\([0-9]*\) .*/\1/g'`

... if i used bash, but i need this under windows :O(

As the last resort i will try to use bash under windows, but i decided
to ask --- please, are there other possibilities?

Thanks!

-- 
  Dmytro O. Redchuk