Astra strikes for about 2.5x the power of a normal hit (.5 times 5), which is only slightly less than a crit. And, if any hits of Astra crit, that's a hit of 1.5x strength, so you add 1 to that final figure of 2.5 to get your rough estimate. Pretty neat. With a character like Gaius, you will be hitting an average of 4x or so each time Astra goes off.
However, equipping Astra, Lethality, and Sol means that each has an independent chance to trigger. I think the one you equip higher up has the odds calculated for it first, so equip Lethality, then Astra, then Sol, so they appear descending.