When to add more to already positive stocks

I have a couple of shares in VOO but I want to add more. Im only up 10% in total returns since ive started (holding 5 shares currently, seeking to add another 5). However, VOO has been hitting a new all time high. Not sure when would be the right time to add more, dont want to skew my average cost too greatly.

Whats your strategy for adding to a position? Would you hold off on pulling the trigger and wait for the next unknown pullback?