"If you don't know where you are going, you might wind up someplace else." [Yogy Berra]
Which rational number is a good proxy of π (3.1415926...)? Enter in cell A1 ‘=pi()', in cell B1 your maximal denominator (for example 10), and in cells C1:D1 ‘=NRN(A1,B1)' as array formula (with CTRL + SHIFT + ENTER). You will get in C1:D1 22 and 7. That means: 22/7 is the nearest rational number to π with a denominator not higher than 10. For 1000 in B1 you would get 355/113.
This algorithm does NOT necessarily find the nearest rational number to a given floating point number with a given maximal denominator and a maximal absolute error. The good message is, though, that it would then return a #NUM! error. In this case please try an individual maximal absolute error.
The author's (Oliver Aberth) original intention was to support exact computation with rational numbers, for example solving a set of linear equations with rational coefficients.
Please read my disclaimer.