Merge #15459: doc: add how to calculate blockchain and chainstate size variables to release process

eb4c43e doc: documents how to calculate m_assumed_blockchain_size and m_assumed_chain_state_size on the release process. (marcoagner)

Pull request description:

  Regarding [this](bitcoin/bitcoin#15183 (comment)) on bitcoin/bitcoin#15183.
  Added an "Additional information" section for this which seems reasonable to me but may not be the best place for this. Also, let me know if anything else should be documented here (like more details).

ACKs for top commit:
  laanwj:
    ACK eb4c43e

Tree-SHA512: 7e6fc46740daa01dd9be5a8da7846e7a9f7fa866bf31fdc2cb252f90c698cfd6ef954f9588f7abcebda2355ec2b2a380635e14a164e53e77d38abefa3e2cc698

Author: laanwj

doc/release-process.md

@@ -24,7 +24,7 @@ Release Process
 ### Before every major release
 
 * Update hardcoded [seeds](/contrib/seeds/README.md), see [this pull request](https://github.com/bitcoin/bitcoin/pull/7415) for an example.
-* Update [`src/chainparams.cpp`](/src/chainparams.cpp) m_assumed_blockchain_size and m_assumed_chain_state_size with the current size plus some overhead.
+* Update [`src/chainparams.cpp`](/src/chainparams.cpp) m_assumed_blockchain_size and m_assumed_chain_state_size with the current size plus some overhead (see [this](#how-to-calculate-m_assumed_blockchain_size-and-m_assumed_chain_state_size) for information on how to calculate them).
 * Update `src/chainparams.cpp` chainTxData with statistics about the transaction count and rate. Use the output of the RPC `getchaintxstats`, see
   [this pull request](https://github.com/bitcoin/bitcoin/pull/12270) for an example. Reviewers can verify the results by running `getchaintxstats <window_block_count> <window_last_block_hash>` with the `window_block_count` and `window_last_block_hash` from your output.
 * On both the master branch and the new release branch:
@@ -369,3 +369,23 @@ bitcoin.org (see below for bitcoin.org update instructions).
   - Optionally twitter, reddit /r/Bitcoin, ... but this will usually sort out itself
 
   - Celebrate
+
+### Additional information
+
+#### How to calculate `m_assumed_blockchain_size` and `m_assumed_chain_state_size`
+
+Both variables are used as a guideline for how much space the user needs on their drive in total, not just strictly for the blockchain.
+Note that all values should be taken from a **fully synced** node and have an overhead of 5-10% added on top of its base value.
+
+To calculate `m_assumed_blockchain_size`:
+- For `mainnet` -> Take the size of the Bitcoin data directory, excluding `/regtest` and `/testnet3` directories.
+- For `testnet` -> Take the size of the `/testnet3` directory.
+
+
+To calculate `m_assumed_chain_state_size`:
+- For `mainnet` -> Take the size of the `/chainstate` directory.
+- For `testnet` -> Take the size of the `/testnet3/chainstate` directory.
+
+Notes:
+- When taking the size for `m_assumed_blockchain_size`, there's no need to exclude the `/chainstate` directory since it's a guideline value and an overhead will be added anyway.
+- The expected overhead for growth may change over time, so it may not be the same value as last release; pay attention to that when changing the variables.