I'm not sure the OP is correct as my main base (Red Rocket) is massively over defended and I'm attacked daily, true only one big attack (when I was returning in a vertibird) but a few raiders are always tring there luck.......generally they are dead before I even make it outside to see what is happening.

That confirms it's food+water, as I thought that warning sign was a bug.

I've only been present for a gunner attack of 4... Codsworth killed one while the rest I got myself. Two of 'em almost killed a turret.

Settlers crowd around the siren, which I never expected, and it actually kind of made the vantage point is was on convenient.

Does anyone know how settlers' weapons works? They usually come with a pipe pistol, but I don't think I've had to resupply any of them ammo. Would this mean that if I gave them better weapons, they'd be better equipped?

Well I built all my settlements with this "fact" in mind and I still get attacked from time to time, In fact I had one settelment that had a defense in the 60's with a total of food and water about 20. I got the message saying they were under attack, I fast traveled there and by then it was all over except for looting the dead Raider bodies

I not sure how accurate that is. my Sanctuary has never been attacked to my knowledge, there was a while were the settlers said we showed those raider. I have it set up as a money maker: Aka i have 6 level 3 trading stands for merchants and producing 122 or 140 water, I collect about 100 purified water every time and sell it off with my collected stash. Use the extra caps for either more vendors or by all junk and throw it into the workbench.

My defense is only like 60-70 and my food is 20 or so (corn, tatos and mufruit= vegetable starch)

The only thing i can maybe think of why i haven't been attacked is i walled off the entire workable area, aka there is a fence along the green border. The only entrance is by the bridge along with all my defense.

So water + food - defense = likely of attack vs number of attack

130 + 20 - 60 = 90 / 0

I think so. All my settlers in Sanctuary are packing shotguns with 12 rounds each, and after several raids I've yet to have to restock anyone. (Now I just need to get them armor)

I have not had any attacks on Sanctuary.........yet.

I have no defense walls up just 4 turrets, perched on the corners of the Workshop House

However I only have the original 5 people and enough food and water to keep them happy for now.

Sorry, but it's not true. The only settlement I've put stock into, so far, is Sanctuary, and I've been hit twice despite having a food/water:defense ratio of 1:3!

Both times were Raiders. Both times, defenses didn't even break a sweat to take them out.

Those laser cannons are wicked accurate.