Open your ide, search for the file name of that xml that you have opened. There should be a class that parses these data in some entities. Those entities should hold information about the rate, if not then try to find usages of those entities and check where this rate is being calculated.
If someone has already attacked the monster, he should be on it's aggrolist. If he's not (well right click -> delete project) but you can also check if there is an attackedBy list.
final DecimalFormat decimalFormat = new DecimalFormat("0.00");
msg.replace("%crtdmg%", decimalFormat.format(your long calcStat here));
you can always change the format
On Interlude client you can edit optiondata_client-e.dat. Normally it requires server side edit too, but since all the information that you give is a "change a client description", i'm assuming that you already did.