They have a team OPS of .597 while the NL average is .735. So they have a rate of 81 (.597/.735 = .81, which is 81 when multiplied by 100). The table below shows the ten worst seasons in OPS relative to the league average in both the NL and AL since 1900. Data from the Lee Sinins Sabermetric Encyclopedia.
But the news is even worse for the Astros. In April, their team OPS was.609 while the league average was .737, So they had a rate of 83.3. In May, their team OPS has been .578 while the NL average is .732. That gives them a rate of 79. So they are going in the wrong direction.