Fuel reserves on voyage

What is the rule of thumb for fuel reserves on a voyage ? 20% for distance run /fuel used is the standard I have always heard .
Is any thing written on this in CFRs , by IMO, ABS etc ?

Refer to stability book and any underwriters requirements. Many ocean towage plan approvals will state required reserves.

I do not think you will find direct reference to min fuel in IMO regs, and not in abs rules. Indirect reference may be thru ISM abt planning for hazards… But an amount or % not dictated to my small knowledge

I second +A465B’s answer. It is as much about stability as it is about how much fuel you need. Check out the stability book and don’t forget that you may be transiting through several different zones (Tropical, etc.) during your ocean transit.