Calculating CR's of NPC's mounted on monsters

[Kealios]

I have a question that has come up while creating an encounter for my PC's:

6 lvl-10 PC's will soon be encountering some scorpion-riding bandits in the desert. I made the NPC's lvl 6, and each is mounted on a CR3 Large Scorpion.

A 6+4 encounter is defined as a EL7, and a 5+3 is a EL6, but I've overstepped that with the CR6 fighters and CR3 scorps. 3x CR7 is a challenge for a lvl 10 group, and with 6 PC's, I'd increase that by 50%...but that's still really only 4 NPC's.

I had toyed with the idea of "combining" the CR's, such that one rider and his mount is CR6, and every other on added would be a CR7 (so 4 marauders would yield 2x CR6 and 2x CR7). I suppose I could also just rate it at EL10 for the NPC's (4x CR6), and a single EL7 encounter (4x CR3).

XP from a EL10 and EL7:
EL10 is worth 3,000 xp to a level 10 party, and EL7 is 1,000 = 4,000

XP from a EL8 (2x CR6) and EL9 (2x CR7):
1,500 and 2,000 = 3,500

How would you rate this?

Thanks in advance,
Kealios

 

[Obscure]

Keeping in mind that estimating EL's is far from an exact science (despite what all the online EL calculators and formulae would have you believe), I would eyeball this encounter at about EL 10.5, technically a 10 bordering on an 11. The exact value of the EL has no effect on XP -- that gets determined by awarding XP for each individual monster defeated -- so it isn't really important to nail down whether it's a 10 or an 11. The point is that it's an appropriate-level challenge for 10th level PCs.

 

[S'mon]

Like Obscure says. EL sounds to me like base 10 (marauders) + 1 (circumstances) = 11, maybe a weak 11 unless the marauders are a good mix of classes - eg 2 Ftrs a Clr & a Sorc is much tougher than 4 Ftrs. XP per the RAW should be calculated as 4 CR 6 + 4 CR 3, then if you think this doesn't reflect the circumstances apply a multiplier - in this case no more than x1.5, and probably none is needed since level 6s aren't much threat to level 10s in my experience.