How do you guys determine the value of a players initial obligation if the number of players that attend each session varies?

I have everyone entered into a table and roll the D100. If it lands on someone who's not present for that session, I consider it the same as if I'd rolled outside the obligation range. I figure that fewer players are going to have a harder time with the adventure, so cutting them a little slack on the obligation is only fair.

I'd go by the total possibly playing and compare that to the table of suggested values.

I'd have everyone on the table and roll it as Krieger suggested.

I just add up whoever's obligation is sitting at the table and use that to roll.

I always set the base Obligation at 5 per character regardless of how many players are in the game, and I don't allow extra Obligation to he taken at character creation. It works for my games, but I also grant extra starting credits and XP depending on the nature of the campaign.