You can do it easily in awk, but your numbers seem to be off by 0.02 (see below for reason). For instance, you can do:

awk '
  FNR==1 {                    # if first record
    printf "DP\tRatio\n"      # output heading
    next                      # skip to next record
  } 
  {
    nf = $NF                  # save copy of last field
    gsub (/[,:]/, " ", nf)    # replace "," and ":" with " "
    split (nf, arr, " ")      # split into arr on space
    printf "%s\t%.2f\n", arr[4], arr[3]/(arr[2]+arr[3])  # output result
  }
' file

Where for example with record 2, arr[3] == 1125, and arr[2] == 2029. The result of the computation is 0.36 not 0.34? (you used 2092 in your calculation instead of the actual 2029 and 553 instead of the actual 533 — mystery solved)

Example Use/Output

You can just paste the script into an xterm with your data file (named file or whatever you change the filename to) as:

$ awk '
>   FNR==1 {                    # if first record
>     printf "DP\tRatio\n"      # output heading
>     next                      # skip to next record
>   }
>   {
>     nf = $NF                  # save copy of last field
>     gsub (/[,:]/, " ", nf)    # replace "," and ":" with " "
>     split (nf, arr, " ")      # split into arr on space
>     printf "%s\t%.2f\n", arr[4], arr[3]/(arr[2]+arr[3])  # output result
>   }
> ' file
DP      Ratio
3154    0.36
883     0.40

So the output based on the file with the fields split as shown above is:

DP      Ratio
3154    0.36
883     0.40

Creating an Awk Script

Creating a script with awk to run from the command line is a convenient way to make the awk commands reusable. For example you can create a file say, splitrec.awk containing:

#!/bin/awk

FNR == 1 {                  # if first record
  printf "DP\tRatio\n"      # output heading
  next                      # skip to next record
}
{
  nf = $NF                  # save copy of last field
  gsub (/[,:]/, " ", nf)    # replace "," and ":" with " "
  split (nf, arr, " ")      # split into arr on space
  printf "%s\t%.2f\n", arr[4], arr[3]/(arr[2]+arr[3])  # output result
}

Then your use of the script becomes:

$  awk -f splitrec.awk file
DP      Ratio
3154    0.36
883     0.40