Volume gives a lot of meaning to a move. It is the easiest, safest measure of a move’s strength. Thus when you see a key resistance level broken on volume you can take more comfort in that being a solid move with enough backing to make it stick. If volume is not there and a stock continues to move higher, that indicates there are more buyers than sellers, but it also shows a lack of conviction on the move and it is thus subject to upset on any adverse news; there were not that many buyers to push it higher (a simple majority), and it would be easy to overwhelm them on some negative news. This move can continue for days; that does not mean we cannot play the move using stocks that are moving on stronger volume, but we have to be cognizant of the next resistance level as that will be serious resistance without a lot of backing. Again it can be pushed back down at that point.
There is no real percentage of move in that circumstance that makes the move safe. The more distance it puts between it and the former resistance provides a better cushion on a pullback or test of the move. If you have low volume on the move up, having more cushion above that support is good. That is why we have been wanting the indexes to put some distance between them and that potential support even if the volume was not good. It is all about being able to consolidate the next move while holding key support and then rallying further. Stronger volume gives you confidence to put more money to work on the move because there is a better chance the move will be stronger on the upside and that the test back will hold at support or even higher.