Sector caveat
Quarterly burn = OCF + investing CapEx. Runway months = cash + short-term investments divided by average monthly burn. Treat clinical-trial milestone receipts as separate (they distort burn for one quarter).
Why R&D is not added back like D&A
R&D is a cash expense in the period it is incurred (GAAP requires expensing). Unlike D&A, it is not non-cash, so the FCF walk does not add it back. This produces FCF figures that look catastrophic but accurately reflect cash leaving the business.
When FCF becomes useful
Post-approval and post-launch, once revenue exceeds direct cost and royalty obligations. For most biotechs that is 8-12 years post-IPO, if at all.