Wow, that's going to be so cool! For accuracy, I would absolutely recommend a refractometer. I have one for my SW tanks and it makes my hydrometer look like a bad joke it is so inaccurate. The good Drs. have one for around 50 bucks, I think, and there's always Ebay.
*edit*
What SG are you going for- I'm trying to remember but isn't brackish somewhere around 1.010? I think the easiest way to do this would be to mix up some SW, and when you top the tank off for evap, just top it off with SW. That is sure to raise SG nice and slow, right? You don't want to do it too fast, as I'm sure you already know. And grab that refracto! lol.
awesome! Thanks for the heads-up!
I've been doing it that way... about every other day I add about 5g of salt water... the sg in the bucket is about 1.030... according to deep six.
I'm not too sure but I think the sg I'm going for is 1.012-1.015