I'm not a fan of doing something like this (pop up a small window, type in a number, etc.) as it's too easy to have lots of windows open all over the place.. There's various ways to do similar manual damage application already in Fantasy Grounds. What I may do is look into allowing addition/subtraction by directly dragging from the modifier stack (under the chat window) - currently to make this work you need to drag to the chat window, then from the chat window to wound field in the CT.

