Hi A B,
Really it depends how the OM structure is set up but doesn't sound simple to me. Therefore, I would recommend to build a custom table to store the approval level, limits, etc and then use that in your custom WF to determine the approvers.
Couple of post which should throw some more light on this issue:
Travel Request/Trip Approval at 2 Levels | SCN
WF TRIP - Travel approval levels and expense | SCN
Hope this helps
Ankur