New chars will start play with XP = Party Average XP (adjusted back up for any LA buyoffs) - Last Level's XP, with a minimum ECL of 3rd.
This will have a new character start about a level behind, and also accurately handle when the average party level is close to leveling, or just recently leveled up.
Let's say there's 3 characters in play, and their XP total is 10,000 (5th) + 11,000 (5th) + 7,000 (4th).
Average XP for that party is 9333. That's 4th level. The last level was 3rd, and to get from 3rd to 4th takes 3000 XP, so, you would start with 9,333 - 3000 = 6,333 XP. That's also 4th level in this case.
If the character with 7,000 XP had previously bought off a Level Adjustment… let's say they were a Tiefling, so they paid off their LA +1 for 3000 XP back when they hit 3rd level… well in that case we'd consider the party average XP a bit higher since we don't penalize you for that: (10,000 + 11,000 + 10,000) / 3 = 10,333. So you would start with 7,333.
NOTE: the average party XP is not protected from losses by crafting magic items, casting spells with an XP component, character death and subsequent Raising, or most other ways to lose XP.