(8*r8+7*r7+6*r6+5*r5+4*r4+3*r3+2*r2+r1)/36
Use standard rounding rules
Which is nice and simple. Basically, "r8" is your highest level ring, "r7" is the next highest... etc etc etc and "r1" is your lowest.
With this, we can begin calculating interesting things about how adding orbs to your rings affects overall CL. smile
For starters, although it appears to have a bias for the highest CL ring on person, this formula actually discourages that practice because of the increased number of orbs required to increase a ring by a single level. Let's compare briefly, shall we?
You have 450 orbs and eight CL:1.0 rings. What overall CL would you rather achieve?
Let's put all the orbs into one ring. You now have a CL:10.0 ring and seven CL:1.0 rings, making your overall CL simply 3.0
Let's put as many orbs between two rings as you can. Two CL:7.0s with 30 orbs leftover, so let's raise one ring to 7.4 and leave the other at 7.0 ; what's our overall CL? 3.6 ; that's a higher overall CL.
Using the level-up strategy I've suggested I've suggested in the past nets you seven CL:4.0 rings and a single CL:3.0 ring. Your overall CL? 4.0
That's a brief outline on why you don't want to focus too much on a single ring. Another way to look at things is via this nifty table I made:
Single: R1 R2 R3 R4 R5 R6 R7 R8 Orbs:
CL n.1: +0.3 or +0.3 | | | | | | | (3 vs 3) +0
CL n.2: +0.7 or +0.7 | | | | | | | (7 vs 7) +0
CL n.3: +1.2 or +1.0, +0.2 | | | | | | (14 vs 12) +2
CL n.4: +1.6 or +1.0, +0.7 | | | | | | (22 vs 17) +5
CL n.5: +2.1 or +1.0, +1.0, +0.2 | | | | | (33 vs 22) +11
CL n.6: +2.5 or +1.0, +1.0, +0.8 | | | | | (45 vs 28) +17
CL n.7: +3.0 or +1.0, +1.0, +1.0, +0.5 | | | | (60 vs 35) +25
CL n.8: +3.4 or +1.0, +1.0, +1.0, +1.0, +0.3 | | | (76 vs 43) +33
CL n.9: +3.9 or +1.0, +1.0, +1.0, +1.0, +1.0, +0.2 | | (96 vs 52) +44
CL n+1: +4.3 or +1.0, +1.0, +1.0, +1.0, +1.0, +1.0, +0.6 | (115 vs 66) +49
Here's the way it works... let's say your overall CL is 1.0 and you want to know what it'll take to reach CL 1.1 ; or you're CL 4.0 and you want to reach 4.1 ; turns out, the math is the same for each level in terms of the number of levels you have to add to each individual ring. In other words...
Seven CL 1.0 rings and a CL 1.3 will give you 1.1
Seven CL 2.0 rings and a CL 2.3 will give you 2.1
Seven CL 3.0 rings and a CL 3.3 will give you 3.1
Seven CL 4.0 rings and a CL 4.3 will give you 4.1
In other words, the only thing about the math that changes at each level is simply how many orbs it takes to increase the CL of a ring by 0.3. However, if instead of spreading out your orbs, you put them all into one ring, that's what the "single" column represents. To help compare, the right-most "orbs" column compares how many orbs it takes.
So, in order to reach an overall CL of 2.0 when you're 1.0, it'll take 115 orbs if you put them all into one ring, but only 66 if you spread them out.
Note that this strategy isn't necessarily the most efficient use of orbs. I've got some thoughts in that regard...