benchmark: add calibrate-n script

This script should help identify the best N
when creating/updating benchmarks

Signed-off-by: RafaelGSS <rafael.nunu@hotmail.com>
PR-URL: https://github.com/nodejs/node/pull/59186
Reviewed-By: Vinícius Lourenço Claro Cardoso <contact@viniciusl.com.br>
Reviewed-By: James M Snell <jasnell@gmail.com>

doc/contributing/writing-and-running-benchmarks.md

@@ -9,6 +9,7 @@
   * [Benchmark analysis requirements](#benchmark-analysis-requirements)
 * [Running benchmarks](#running-benchmarks)
   * [Running individual benchmarks](#running-individual-benchmarks)
+  * [Calibrating the number of iterations with calibrate-n.js](#calibrating-the-number-of-iterations-with-calibrate-njs)
   * [Running all benchmarks](#running-all-benchmarks)
   * [Specifying CPU Cores for Benchmarks with run.js](#specifying-cpu-cores-for-benchmarks-with-runjs)
   * [Filtering benchmarks](#filtering-benchmarks)
@@ -142,6 +143,46 @@ buffers/buffer-tostring.js n=10000000 len=1024 arg=true: 3498295.68561504
 buffers/buffer-tostring.js n=10000000 len=1024 arg=false: 3783071.1678948295
 ```
 
+### Calibrating the number of iterations with calibrate-n.js
+
+Before running benchmarks, it's often useful to determine the optimal number of iterations (`n`)
+that provides statistically stable results. The `calibrate-n.js` tool helps find this value by
+running a benchmark multiple times with increasing `n` values until the coefficient of variation (CV)
+falls below a target threshold.
+
+```console
+$ node benchmark/calibrate-n.js benchmark/buffers/buffer-compare.js
+
+--------------------------------------------------------
+Benchmark: buffers/buffer-compare.js
+--------------------------------------------------------
+What we are trying to find: The optimal number of iterations (n)
+that produces consistent benchmark results without wasting time.
+
+How it works:
+1. Run the benchmark multiple times with a specific n value
+2. Group results by configuration
+3. If overall CV is above 5% or any configuration has CV above 10%, increase n and try again
+4. Stop when we have stable results (overall CV < 5% and all configs CV < 10%) or max increases reached
+
+Configuration:
+- Starting n: 10 iterations
+- Runs per n value: 30
+- Target CV threshold: 5% (lower CV = more stable results)
+- Max increases: 6
+- Increase factor: 10x
+```
+
+The tool accepts several options:
+
+* `--runs=N`: Number of runs for each n value (default: 30)
+* `--cv-threshold=N`: Target coefficient of variation threshold (default: 0.05)
+* `--max-increases=N`: Maximum number of n increases to try (default: 6)
+* `--start-n=N`: Initial n value to start with (default: 10)
+* `--increase=N`: Factor by which to increase n (default: 10)
+
+Once you've determined a stable `n` value, you can use it when running your benchmarks.
+
 ### Running all benchmarks
 
 Similar to running individual benchmarks, a group of benchmarks can be executed