1993 Chevy Silverado minimum valve spring

This question has many answers. First I need to know what you are doing to the engine. Are you rebuilding it? Are you rebuilding the heads? Are these stock valve springs? Is it a stock cam?
Normaly, if there is any question about the strength or integrity of a valve spring, you should replace the whole set.

Is it a steel head or aluminum head? Are the valve seats stock? Are the valves steel or titanium? What is the compression ratio? Are you using stock rocker arms? What is the valve duration of the cam?
What I'm trying to get at is this, there is not an exact answer to your question until you tell me what you are doing to the engine. In order to find the minimum acceptable pressure for the spring you must consider what you are asking the spring to do; and are the parts that support the spring--like the retainer -- the keeper -- the head material etc, capable to do the job. There is no reliable answer to your question until all the questions above are answered.