Hey, I'm glad they're flourishing! You could do a couple things to slow down the amount you are getting. If you use 1% or 2% milk, it will take longer to thicken up. You can also reduce the amount of grains you use each time- when you strain it out, take only 2-3 T of grains & use those to culture your next batch. It will take longer to culture milk with less grains. The leftover grains can be stored in the fridge, in a bit of milk. The grains will also grow faster in warmer weather, they really multipliy like crazy. I wouldn't strain out half, personally- the remaining half will never get drank, it will just culture over & over, becoming really sour & yuk! You certainly could- try it & see how you like it. But starting with fresh milk seems better than recycling the cultured stuff.
I use about 1/8 cup of grains to about 20 oz. of 1% milk- I let it culture about 48 hrs. When that batch of grains starts to grow out of control, I dump it in the strainer, dump in the reserved grains from my fridge, & sort of mix 'em all up- then I just take out 2-3 T of grains & start all over again. I have a BIG jar in my fridge, nearly filled with grains.