maybe it will help in steps:
1) get your current KH to 4.0 (without doing any water changes in the meantime). Do this by gradually adding b.s. to the water over a couple day period of time.
2) when doing a 50% water change, either: a) bring the water you will be using in the waterchange to 4.0, instantly (i.e. if you are doing a 5g water change of 0dKH RO water, you will add 4/10 teaspoons of b.s. right then, mix it in, and then dump into the tank), or b) fill up the tank with the water, and then add in the 4/10 teaspoon into the tank.
fyi, the 4/10 teaspoon is calculated as 1/10th teaspoon to raise KH by 1dKH for 5g's. You need to rasie the dKH by 4, so 4 x 1/10 = 4/10t. This is also assuming that you are using 0dKH water (i.e. RO, or very very soft tap water).
Don't worry about being a little off from 4.0. really anywhere between 3.5 and 4.5 is where you need to be aiming. it is better to just be consistent anyway.
edit>> you do have a 10g tank right? thought I read that in the posts, but cant find it now... what size tank is it?
450g Planted Discus Tank - Work in Process!