diff --git a/README.md b/README.md
index 250463e..028d7ed 100644
--- a/README.md
+++ b/README.md
@@ -123,8 +123,11 @@ What the [upaas](https://git.eeqj.de/sneak/upaas) app for pixa needs:
       comma-separated; default none. Every image on a listed host is served to
       anyone without a signature, so list only hosts you control, where nobody
       else can put images.
-    - `PIXA_CACHE_MAX_BYTES`: disk cache limit in bytes; `0` disables it;
-      default 75% of (free space + what the cache holds)
+    - `PIXA_CACHE_MAX_BYTES`: disk cache limit in bytes; `0` disables it. Set
+      it: the volume is a directory on the host's filesystem, shared with the
+      host and every other app, and the default, 75% of the sum of that
+      filesystem's free space and what the cache holds, lets the cache grow to
+      three quarters of the space they all have left.
     - the rest are in the table under Configuration below
 - **Health check:** the image's `HEALTHCHECK` requests
   `/.well-known/healthcheck.json`. upaas reads the container's health 60 seconds
diff --git a/TODO.md b/TODO.md
index c046779..9d74fc3 100644
--- a/TODO.md
+++ b/TODO.md
@@ -30,6 +30,12 @@ P2: security: per-origin rate limiting
 
 # Completed Steps
 
+- 2026-10-15 "Running under upaas" in `README.md` tells the operator to set
+  `PIXA_CACHE_MAX_BYTES` (closes #251): it listed the variable as optional, but
+  under upaas the volume is a directory on the host's filesystem, so the
+  default, 75% of that filesystem's free space plus what the cache holds, let
+  one pixa take three quarters of the space the host and every other app had
+  left. The section now says so. The default is unchanged.
 - 2026-10-15 the example config allowlists no host (closes #249): it allowlisted
   `github.com` and `user-images.githubusercontent.com`, where anyone can put an
   image, so a deployment set up as Getting Started says served those images to