Greg,
The datasheet seems to be for a PVRV (pressure vacuum relief valve). You should "Red Flag" this post and re-post in the Safety Relif Valve engineering forum.
In any case, you should not be relying on eng-tips to size a valve, in particular a safety valve. The purpose of this site is to help, not to do your work.
Also, there are other consideration to size a PVRV that you have not included, such as back pressure constant and variable.
"Do not worry about your problems with mathematics, I assure you mine are far greater."
Albert Einstein
Have you read FAQ731-376 to make the best use of Eng-Tips Forums?