For Battle Armor particularly, one probably should consider certain "breakpoints", weapons they are expected to face. Noteworthy ones are 5, 7, 8, 10 and 15.
Inner Sphere medium lasers are common, and being able to survive some multiple of them is valuable. The original Fenrir's armor (value 5) is arguably a failure as it can only take one medium laser hit before any damage will kill the suit.
For the Clans, multiples of 7 are notable due to their ER medium laser and medium pulse laser having that damage value, though in modern day the Inner Sphere factions also need to be mindful of this.
Depending, people probably want to save bigger guns bigger targets and only utilize medium laser types against battle armor commonly, perhaps with follow-up SRMs or LRMs or LB-X cluster shots. For Inner Sphere, large lasers and light Gauss rifles are common enough it is reasonable to expect those being used against BA as well.
Suit weaponry also matters. If you have more reach than most of your opponents, you can get by with lesser armor. An assault suit with LRMs is reasonably safe. To be sure, big guns like ERPPCs and Gauss rifles are likely to kill them if they have low armor, but BA LRMs aren't terribly powerful so if the enemy is wasting high power shots on your battle armor, your other units are that much safer. Of course actually having reach with battle armor is very difficult, and in many cases your weapons merely match what your targets have.
There is some argument for not bothering to armor assault battle armor too heavily: Infernos. You lose a suit for every infernos that strike battle armor, regardless of their weight class, making assault battle armor prime targets for inferno missiles. This is not a problem if you have access to Fire Resistant Armor, though that is Clan-tech.
Since battle armor cannot reach high Target Movement Modifiers easily, all battle armor do need armor. Speed is not life. Lesser probably armor should be compensated with stealth or mimetic armor, or camo module if possible. Specialty armor types are are also an option, reflective armor for example allows even measly 6 points to survive two Inner Sphere medium lasers for example.
In my opinion:
-Medium combat suits should aim to have at least 8 points of armor, though you can get by with 7 if the expected opponents are using Clan weapons and you get something nice for the freed weight. The Salamander is a good example.
-Heavy suits will usually want 10 or more armor, otherwise you're very much a glass cannon or kind of OmniMech-portable turret/minefield (like the Phalanx for example).
-Assault suits will want minimum of 10 armor, though with minimum-only your suit will be somewhat ambush-oriented. The more firepower your suit has, the more priority target it will be. Battle armor slot limitations also mean you can only so much firepower, so there isn't too much reason to skimp on protection deliberately.
- Should be noted the decision between heavy and assault suit essentially boils down to question: Do you want the suit to be OmniMech-portable or are you fine with being limited to transport vehicles? (This ignoring BV and C-bill cost concerns).
-Spotter, special operations, or utility battle armor can get by with lesser armor (5 or 6 for medium), though they will almost certainly want some kind of stealth to compensate. But even then too little is not good, you don't want your enemy to hunt them because they have low armor. Light battle armor tend to fall into this class automatically.
A special note:
If your suit is slow (planned to be 1MP only), Detachable Weapon Pack allows cutting weapon weight (and slots) down considerably, though it does require giving the suit at least 2MP as DWP cuts that down to 1MP until jettisoned. Particularly useful for quad assault battle armor that get 2MP for free.
