So you get about 10 (a little higher but let's keep it simple) pts per key level per dungeon. Theres 8 dungeons. So at 500 rio, for real quick math, you could do 500/8 = 62.5. So on average each dungeon gave you 62.5 pts, which means if all dungeons were even, you've done all 8 on +6. At lower levels u can jump up 2 or 3 levels at a time, so at 500 rio, id say you're prob fine qing for an 8, maybe a 9. For safe bet, q for 7s and work your way up.
I personally don't care to look at rio until my alts are doing like a +7. Then at that point I make sure you at leaat have some history. At +10, I look to see if you a have ~700+ score, a couple plus 10s, and at least an 8 of that dungeon done. At +12, I care that you at least have 1 lower than the current dungeon done. So a +11 of that dungeon, other keys around the same level, and multiple keys run in that bracket. Thays just how I do it.
For +15, im looking for super smooth runs so honestly at least +15 already done on that dungeon, multiple +15s done total, and at least a 1300 score.
Idk about doable. Theres some ppl that can time a +10 in that dungeon and instantly do the 15. But im not taking the chance. Most ppl pugging past 12 usually try to do all on 12, then all on 13, then all on 14, to slowly build score to look more attractive in the group finder.
Ilvl is another metric too. As well as class and spec