If it really matters that much, find a monster close to what you just made, see what they give as XP, and then give your monster that.
I mean unless you are designing these monsters to be published... then anything you do for your home campaign does not need some sort of precision of any sort. Your players don't know about and don't care about what the CR levels the encounters are that you throw at them, nor do they care about the precise XP each individual enemy had in the fight. You throw some monsters at them, they defeat them, they earn some random total of XP. That's all they care about. How everything is set up "behind the screen" does not matter to them, so there's no reason for any DM to lose sleep over it.