Combat - How are to-hit values calculated?

In my mod I'm tweaking the values of ships to make the big ships bigger, but they should be correspondingly easier to hit (which would be offset by the penalty to hit for equivalent sized weapon mounts). So I want to know, how is net to-hit chance calculated? Is, say, a single 99 defense modifier 10 times better than a single 90 defense modifier, or are modifiers of >100 possible (due to a more adding and subtracting method)?