If there a formula to decide when more decimal places will be added to Bitcoin? I know it is possible to allow for more than 8 decimal places in Bitcoin. We all hope that Bitcoin will become extremely valuable one day and increments smaller than 0.00000001 BTC will be helpful.

Has the decision of when this will occur already codified? Is there any reason why it cannot be decided now a long time before it matters?

It would be nice to solve a potentially important issue like this now in code instead of one day needing to debate the issue after it becomes contentious (like the blocksize today).
To the best of my knowledge this has not yet been codified and it is hard to imagine there being a need to smaller Bitcoin increments anytime soon.

At this point smaller increments would suffer transaction relay problems without the addition of a miners fee far in excess of the Bitcoin being sent.

This could be settled now by scheduling the change based on Bitcoin valuation. However to do that we would need to agree what to measure BTC valuation against? 


Gold perhaps? 
World GDP?
International wire transfer volume?


Setting the benchmark to trigger the change to adding more decimals may lead to more debate.