According to the [url=http://www.bungie.net/7_Hotfix---10142014/en/News/News?aid=12283]1.0.2.2 patch notes[/url], the Vex Mythoclast received a 34% base damage nerf.
While many people felt that 34% is too harsh of a nerf, the Mythoclast actually recieved a near 50% damage nerf.
Before the patch, [url=http://i.imgur.com/7ogECuN.jpg]the weapon did 45 damage at point-blank range in the Crucible[/url] ([url=https://www.youtube.com/watch?v=uZ7U7rV47PA]source video[/url]). There are countless YouTube videos that exist that can verify this.
After the patch, [url=http://i.imgur.com/bmuwydj.jpg]the weapon does 23 damage at point-blank range in the Crucible[/url] (my image). This is verifiable in game.
This is nearly a 50% damage nerf, which is quite of a lot larger than the already controversial 34%.
I have not recorded PvE damage numbers but they seem to reflect this.
I am not giving an opinion on the nerf. I am merely presenting facts.
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Is it possible your doing the math wrong in the first place. We don't actually know how crucible damage is calculated anyway since Bungie is very bad at communicating with the player base on these things or listen to the worst players when they want to nerf things.