119.5% x 2.5 = 298.75mg
105% x 2.5 = 262.5mg
100% x 2.5 = 250mg
91.7% + 9.1% = 100.0447%
If you wanted 250mg you should just add 10%
We don't know what you added besides the extra 5% that would make it 262.5mg
Keep in mind you will most likely not nail these numbers spot on dead nuts. But something is way off.
If you want to play the side of caution do something like this.
91% purity plus 14% more raws is rounded to 103%
103 x 2.5 = 257mg rounded giving you and the test a rounded 3% room for error.