The yellow bar is pretty inaccurate, in that it doesn't account for a lot of things. Using a number would require fixing all of those things. It would be quite nice to have an accurate numerical display, but given the number of corrections that would have to be made to the system in order to provide accurate information, it's pretty low on the list of things I'd like to see happen for that much coding effort.
In other words, yeah, I agree it'd be nice, but it's a lot less easy of a change than it sounds like, so just get used to mathing your total damage in your head.
It doesn't matter if it isn't accurate to the point of calculating opponent's shields and stuff.
The yellow bar as it is right now has as code something like:
Sum the force of all of the opponent's creatures that are able to attack, call this S.
The total HPs you've got are called L.
The remaining HPs are called R.
You need then to compare S with L (since if one has 150 total HPs and one other has 100 total HPs, the same damage will cause a shorter line on the 150 one than on the 100 one), obtaining S'.
You've got to compare R with L (same thing, if one has 99 HPs left on 100 total, and one other has 99 HPs on 150 total it will be different), obtaining R'.
You now subtract S' to R' and obtain the Green bar (if it goes under the zero it will keep its negative number but will not show), this point will be called G.
Now you go back to the original R' and call the point it reaches Y.
Form a bar from G to Y, this will be the Yellow bar.
Now, what I ask is just the first line of code, nothing else.
EDIT: Oh, forgot viruses, but you just have to sum them to the attacking creatures.