I don't think there is an optimal stack size bc so much of it depends on your opponent's tendencies and their stack size as well.  However, there is a floor where 3 betting lighting is unprofitable, and I believe that you accurately stated that is 25BB.  Generally, I agree with your assessment that both you AND your opponent must have > 25BB.  The reason for this is that the players who are the most exploited by the light 3 bet are those who are raising too light themselves.  Generally, they will raise to 2.5BB but fold to even a modest size 3 bet of say 6.5 bb.  If you ever raise 1/4 your stack you are basically committed so 3 betting light with less than 25-26BB is bad.  Once you have 26+ BB then it depends a lot more on your opponent's tendencies than your stack size.  30BB is absolutely fine for 3 betting light against an opponent who opens frequently but has a high (>50%) fold to 3 bet tendency.