If there a formula to decide when more decimal places will be added to Bitcoin?
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.