Use fr instead and you can use it as a drop in replacement for % if you want (It just doesnβt have to add up to 100. You can do 66.6fr 33.3fr or just 2fr 1fr)
You can adjust those values until the rows match the heights you want to have. That is the height of the overlap, the first is the height of the top-right image (because it's the image that's only in the first row), the second determines the height of the bottom left image
Like Z said, 1fr is a relative unit. All the values are added up, then the available space is distributed according to how many fr each row or column is defined as. is the same as , both result in a middle row/column that's twice the size of the other two