With me it's kinda just a "feeling" on the amount. lol
I use slightly rounded 1/2 cups of salt in my mixing water to get to 1.025, it's something you just kinda figure out I think.
To get your water up the extra .001 I would just use salt water for your next few top offs instead of regular ro
water. That's an easy way to adjust for such a small amount of adjustment.