diff --git a/rfcs/0139-longer-identifiers.md b/rfcs/0139-longer-identifiers.md
index 16b9213..434181b 100644
--- a/rfcs/0139-longer-identifiers.md
+++ b/rfcs/0139-longer-identifiers.md
@@ -1,4 +1,4 @@
-# RFC 139 - Increase identifier length from 20 to 44
+# RFC 139 - Increase identifier length from 20 to 38
 * Comments: [#139](https://api.github.com/repos/taskcluster/taskcluster-rfcs/pull/139), [Bug 1520579](https://bugzilla.mozilla.org/show_bug.cgi?id=1520579)
 * Proposed by: @djmitche / @mitchhentges
 
@@ -19,25 +19,42 @@ We can, however, allow more than 22 characters.
 
 We will define two types of identifiers:
 
-* "identifiers" -- up to 40 characters, generally containing human-readable identifiers
+* "identifiers" -- up to 38 characters, generally containing human-readable identifiers
   * `provisionerId`, `workerType`, `workerGroup`, `workerId`
   * `schedulerId`
-  * `organization`, `repository` (in tc-github)
 
 * "slugids" -- exactly 22 characters, always a slugid
   * `taskGroupId`
   * `taskId`
 
+The following remain separate:
+  * `organization`, `repository` (limited to 100 characters in taskcluster-github)
+
+The rationale for the choice of 38 is the [task status routing key](https://docs.taskcluster.net/docs/reference/platform/taskcluster-queue/references/events#routing-key) which contains
+
+| name           | max characters |
+|----------------|----------------|
+| routingKeyKind | 7              |
+| taskId         | 22             |
+| runId          | 3              |
+| workerGroup    | N              |
+| workerId       | N              |
+| provisionerId  | N              |
+| workerType     | N              |
+| schedulerId    | N              |
+| taskGroupId    | 22             |
+| reserved       | 1              |
+
+With a total of 9 dots between each of those components.
+So the total length is `5N + 55 + 9 <= 255`.
+Solving for N, we get 38. 
+
 This change requires changes to schemas and API declarations in services in the monorepo, as well as to `aws-provisioner` and possibly `ec2-manager`, workers, and client libraries.
 Much of this work has [already been done](https://github.com/taskcluster/taskcluster/pull/110) by @OjaswinM.
 
 We already have checks on AMQP routing key lengths, and the increased length does not cause any failures in that code.
 
-# Open Questions
-
-TBD
-
 # Implementation
 
-* https://github.com/taskcluster/taskcluster/pull/110
-* TBD
+* [taskcluster/taskcluster#110](https://github.com/taskcluster/taskcluster/pull/110)
+* [Bug 1520579](https://bugzilla.mozilla.org/show_bug.cgi?id=1520579)