Complex sentence: By the 21st century, sawmills could easily cut 40 or more 2x4 pieces with a cumulative width and depth variation of less that 1/8 in, so the prevailing actual values are closer to 1 3/8 in.
Split into simpler sentences: By the 21st century, sawmills could easily cut 40 or more 2x4 pieces with a cumulative width and depth variation of less that 1/8 in. So the current prevailing actual values are closer to 1 3/8 in.