The 150/190/150 is just a ratio - but the idea in that case would be that you create 490g refreshing dough. After 12h, you take the 290g sponge, and put the remaining 200g back in the jar. If you don't have some left over, you won't have any left for next time!

Then you have 200g left over, and can choose, when you're refreshing, whether you want to refresh 150g next time, or use the 200g, match it with 200g flour, and up it to 253(ish)g water. And your sourdough quantity would keep growing...

If you get in a mess with the baskets (I've got some compressed fibre ones that work great, and are only slightly reminiscent of hospital sick buckets) remember that rubbing flour in will pretty much always get any mess out...