Ammo saving is multiplicative, not additive. So with your setup of a 50% chance to save ammo gun and three 20% chances to save ammo from armor and accessory, you have a total of approximately a 75% chance to save ammo. Having played Terraria to death in 1.1.2 using the Megashark with Hallowed Ranged, I can confirm without a doubt those bonuses multiply, not add. I counted off about 500 bullets and had about 200 less than I started with (I saved around 60% of my bullets). But yeah, if they did add, it would be a game breaker because at a 110% chance to save ammo, you only need 1 shot. But sadly, that's not the case.
I mean I still think having both a 40% damage penalty AND 20% increased mana cost is stupid for Spectre's healing set. The damage set's bonus damage is just too weak to be useful. Kind of like Chlorophyte's stupid crystal. Sounds awesome on paper but in practice the crystal's damage is poor. Same with a lot of other accessories, like the Honeycomb. They're useless unless you're combining them with a Lucky Coin and spawning Mimic to 1 damage spam with a terrible modded Bee Gun, Vilethorn, Slime Staff, etc. Either way, I collected materials for Ichor Flasks since I'm going back to a melee setup.
Also I noticed the Poison Staff got hurt badly in the update, getting both weaker and much more expensive. I'm almost debating if I want to waste 14 Chlorophyte Bars to upgrade to a Venom Staff. I mean I have more money than I know what to do with, and I'm not above Alt + F4 save scumming if I blow 10 Platinum for nothing. I may give ranged a try again with the Pulse Bow. I bought another weapon just for the sake of having it. It's kind of a poor man's Megashark with the same god-awful aim as the Chain Gun, just fires slower. Chlorophyte Bullets are probably a must with it. Then again, with their nerf, I'm pretty much using Ichor Bullets if I do use guns at all anymore.
They never should have made PM/FM so difficult they're near impossible without that caliber of broken healing just to survive. And in the case of FM, even the broken Spectre wasn't enough to keep you alive. You can't wall yourself off from any of the monsters, scumming with the Nurse probably only works once until she's 3-4 hit KO'ed, and remember, you have to make it 45s until your Potion Sickness expires if you get down to using potions. Saving hearts up from the bosses only gets you so far as well.