I'd recommend only using reduced size if you lack the room for a triple or quad turret, and even then to not go below 1HS to avoid the frailty of a 0-HTK components.
Only if you are not going to utilize additional boni. long traking and high quality crew (not even mentioning a jedi-class fighter pilot) could drive to-hit chance of a full-sized cannon well over 100%, so using those on a non-critical vessels would be a waste.
Edit:
Also you calculation example is correct for RoF 1 gauss cannons only. If you take another extreme edge of RoF 5 cannons - the battary of 12 8% barrels will have a 99.3% chance of scoring a hit (60 shots).
For OP:
There could be a situation when enemy is bombarding you with a large number of single-missile salvos (I would be such an enemy) 4 shots out of 5 would be wasted for every full-sized gauss cannon. Smaller barrels may be better distributed to cover more salvos:
Let's assume your ship is attacked by 10 1-missile salvos.
You have 30HS space on the ship for defensive RoF 5 gauss cannons. (I will ignore FC, turreting and any additional boni here for simplicity)
If you decided to use five full-sized cannons - you would destroy 5 missiles, and take a hit from 5 others. so it is a 50% protection.
If you decided to use 10 half-sized cannons - you would engage every salvo, but you chance of hitting any particular missile would be 1-0.5^5 = 96.9%. The chance for you hitting all of them (which was quite impossible in former full-sized setup) would be 0.969^10 = 72.8%
You can now calculate the average damage in the second setup using the formula given by sublight.
That is all just to prove that optimal setup will differ from greatly your situation.