I wouldn't sweat the BHN too much. First of all, unless you have access to lab equipment, it hardly matters what the "the number" is. What matters is approximate or relative hardness.
I agree on keeping it separate, unless maybe you can cook up a huge batch at once. If you mark it well, it will be easier during casting time to guesstimate (and it really is just guessing) what to throw in the pot for your desired hardness. I think too many casters overestimate their 'knowledge' of percentages and proportions and other values that they cannot actually measure.