60%-100% is the minimum for a melee attack in, I believe, all the versions before RMU. RMU drops that to 50%-100%, which doesn't sound like much, but mechanics/balance wise it is a bigger factor that it would appear on paper. Missile attacks run 30%-60% pre-RMU, can't remember if that's different in it.
The one item I can think of that has over a 200% activity is reloading a Heavy Crossbow. So the amount of activity is requires means you're loading for two rounds, part of the third, but can still fire it in the third round. The reason you'd assign a percentage to that (and anything really) is because things like the skill Adrenal Speed and speeds Speed and Haste will give you more activity, sometimes even different amounts of activity, so it's easier just to do it in percentages. If you only ever performed one action per round then I could see just labeling time factors in rounds.
Thanks for the explanations Cory
In the end I decided that the activity system is just not helpful so I killed it. I didn't use CEATS either because my players couldn't get into the idea of breaking down their actions by time, and I don't blame them as the tendency then just becomes doing as many simultaneous actions as you can get away with (I run up to the brigand while drawing my sword and grabbing my henchman up off the ground with my off hand) which means constant common sense adjudication.
So I think I will just do Common Sense Initiative for the most part, and solve disputes with a single Init roll per combatants. I like the idea of Military Initiative too: the idea that whoever is winning at that moment has the initiative, they have the momentum in the fight and the struggle is to regain that momentum. When you look at it that way the artificial mechanic of an Init order can be chucked and it becomes more about
who hit versus when they attacked. I absolutely hate Initiative order description:
"Ok next is Rognor (after Orc A who hit him square in the head with a sword)."
Rognor Player: "I'm going to attack Orc A."
GM: "Ok roll your attack"
Orc A survives
GM: "Ok now its time for Blatno the Mage"
etc.
I dislike this because you get a wait your turn feel that to me is extremely boring and hard to mesh with what is essentially a simultaneous calamity. It also encourages wargaming, as you are now a piece in a line, and players will act accordingly.
There is a game called Dungeon World that has no initiative, there is a description that can result in combat and if so, one roll determines:
low roll) Player fails and is hit.
mid roll) Player and Opponent both take some damage.
high roll) Player hits opponent does not.
I have thought about trying to apply this idea to Rolemaster, but I don't know how it could work. I love the OB/DB and the hit tables in RM so much that I won't use a different system, but old school linear initiative systems no longer entertain me.