[Radek]

The computations are simple:

1. Each character in your party gets an equal share of XP.
2. Each active class of a particular character gets an equal share of the character's amount of XP.

Examples:
1. A solo fighter/mage/thief reaches the same levels in his classes as each member of a party of three dual class characters - because the FMT gets all XP and divides them between his 3 classes while each member of the party gets one third of XP which he does not divide further.
2. A solo sorcerer with Aerie and Yoshimo gets one third of XP. Aerie gets another third which she divides between her 2 classes and Yoshimo gets the last third. After dropping Yoshimo, your socrerer will get one half, Aerie - mage will get one fourth and Aerie - cleric will also get one fourth.

Therefore, estimate the total amount of XPs available, catch your calculator, and see what you can reach with a particular party.