To err is human, to forgive divine.

Alexander Pope An essay on Criticism.

You’re only as good as your last mistake.
This saying is a warning against pride and resting on one’s laurels.

Them as don’t, don’t make mistakes.
A colloquial way of saying, “If you don’t do anything, you won’t do anything wrong”.  What about sins of omission?

To rule something out means that it is not considered as a possibility.  The expression derives from crossing some words out with a ruler.  There is a facility for doing this on WordPress – like  this. 

No mean uses the sense of average.  He’s no mean athlete – he’s a very good one.

A mean is an average; the means are the resources; mean as an adjective maybe spiteful or miserly.  If you need any more meaningsI suggest you consult the dictionary!

Even out may mean average out.

The acid test is a cliché from chemistry.

“Quite so!” used to be a popular way of agreeing with a statement.  Nowadays “Exactly!” is more frequently heard.  Quite means completely or fairly depending on the context.  So has the meanings thus or in this manner (among others).

Get the lowdown means find out the gossip or latest news.

Back to basics means starting at the beginning.

Back to the drawing-board is about reviewing a design or plan.  Before computers and computer-aided design (CAD) draughtspersons carefully drew engineering components.  These were then manufactured with the measurements from the blueprints The expression back to the drawing-board is used, when something is not working and has to be rethought.

From first principles is a phrase from mathematics and science.  It involves going back to the basic ideas and working something out from there.  Someone once commented that I work from first principles.  (I tend not to do things from habit, but think about the best way to do anything.)

On principle means from one’s beliefs.  (I do not shop on Good Friday on principle.)

The best part of may be applied to quantity, when it means most or the majority.  We’ve had the best part of the day is about the quality of the weather.

Nuts and bolts are ironmongery used to hold machines together.  The phrase is a metaphor for the basics or essentials.

The exception proves the rule.  Brewer explains that if there were no rule there could be no exceptions. 

Exceptio probat regulam.

( Columella)

A benchmark is a standard or point of reference.  The Oxford Concise Dictionary explains that surveyors cut marks in rocks at the same level.

The best known rules in the Bible are the Ten Commandments. (Exodus 20:1-17)

Did you know that God wrote them on stone tablets?  Why he had to do this for the second time is explained in Exodus 33-34