How is the current format for an MTG booster draft calculated?
Wizards of the Coast designs sets for specific Limited environments. 

The intended draft format for Lorwyn/Shadowmoor block was:


Lorwyn draft: 3x Lorwyn
Morningtide draft: 2x Lorwyn, 1x Morningtide
Shadowmoor draft: 3x Shadowmoor
Eventide draft: 2x Shadowmoor, 1x Eventide


Cards from the two halves of the set were not intended to be drafted together.

The MTG Tournament Rules specify the officially-recommended booster mix for current sets (see Appendix D).