Ok, here is a differentiated reward system on the same model (first level one session, second level two sessions, etc). Since xp required in 3E is linear, with each level requiring 1000 new xp times your character level, the easy way to make it progressively slower is to divide all rewards by character level.
* Use the 3.5 xp tables
* Assign xp exactly as normal, but multiply it by four, then divide the reward by character level.
Or
* Assign each session a reward of 1000 xp. Divide this reward between the challenges encountered in this session, giving 100% to the guy who fought all of them, 50% to he who confronts half of them, and so on.
Why not just break it down challenge by challenge, as follows:
Challenges this session (of whatever type) were W, X, Y, and Z
Party members are A,B,C,D,E, and F (note: character E is a notorious coward who always tries to be elsewhere when danger is near)
Challenge W is worth 400 ExP total, characters A,C,D,F took part, so 100 ExP each.
Challenge X is worth 250 ExP total, char's A,B,C,D,F took part, so 50 each.
Challenge Y is worth 480 ExP total, only char's B,C and F got in, so 160 each.
Challenge Z is worth 1500 ExP total, everyone got in, so 250 each.
So, the totals by character for this session are:
A = 400
B = 460
C = 560
D = 400
E = 250
F = 560
C and F took the greatest risks (got involved in all the challenges, intentionally or otherwise) and got the greatest reward. Character E gets the least; the only challenge he helped out in was one he could not avoid.
All this requires is some note-taking during the session as to who gets involved with what. I use a simple chart with the characters' names down one side and the encounters along the top written in as they happen; I put little ticks as to who got in on each encounter, and work out the actual ExP amounts later. (actual system is vaguely 1e, but this works for any edition)
In an all-in system here each character would have received a flat 2630/6, or about 438 each - I maintain this is grossly unfair to characters C and F and at the same time is giving character E rewards it does not deserve.
I don't know why, but I had figured you for an old-school DM, and what you condemn here is pretty much the old-school approach; players are presented with a problem, and any approach that solves the problem merits a reward. Seems I was wrong. My players are pretty much the opposite of yours - they compete to be the most heroic, hence different experiences and rules.
I am an old-school DM. That said, you're quite right: any approach that solves the problem merits a reward...but only for those who helped solve it, by whatever means. A character who sleeps through an encounter, or who is off doing something else, doesn't help solve anything and should therefore not get ExP for it.
Lanefan