Replacing numbers in bash scripts

Hi,

I have lines in some files that look exactly as below, and the line numbers they occur in are always the same. (Lines 136-139)

W 0.00000000 0.00000000 2.00000000
W 0.50000000 0.50000000 2.50000000
W 0.00000000 0.00000000 3.00000000
W 0.50000000 0.50000000 3.50000000

I'd like to replace the last number in each row for each file by numbers related to the variable $vacheight, so that the median of these 4 numbers is always half of $vacheight.

The files are all named "run_example", but are in different folders named "examples#" where # is a number from 140 to 199.

As a related example, Line 117 reads:

  celldm(3)   = 5.000,

Below is the relevant section of code that works for Line 117 and increments the number on Line 117 by 0.1 each time, in a different folder.

vacheight=`expr 4.500`
for i in */run_example; do
    printf '117s/= [^,]*/= %s/\nwq\n' "$vacheight" | ed -s "$i"
    vacheight=$(echo "scale=3; $vacheight + 0.100" | bc)
done

Thank you very much for your help!