diff --git a/bsip-0040.md b/bsip-0040.md
index a33801e..ccd10d2 100644
--- a/bsip-0040.md
+++ b/bsip-0040.md
@@ -180,7 +180,8 @@ Note:
 Adding a custom active authority means increased effort for the backend, and with a stateful one also the need for more storage. Proposed transaction fees:
 - `install_custom_active_authority`: Normal accounts can only create custom active authoritites with a duration of maximum 1 year, LTM can do any duration. Two options come to mind:
   - A fixed high fee independent of authorities content
-  - Tied to the duration and complexity of the custom active authority. Transaction fee is then `fee = flat_fee + basic_fee * duration` where `basic_fee` is calculated according to complexity (e.g. size of authority, number of restrictions and etc.). Fee is capped at 1 year for LTM.
+  - Tied to the duration and complexity of the custom active authority. Transaction fee is then `fee = flat_fee + basic_fee * duration` where `basic_fee` is calculated according to complexity (e.g. size of authority, number of restrictions and etc.). Fee is capped at 1 year for LTM, `duration` refers to the time that is still left, not the actual duration 
+  (i.e. `date_to - max(now, date_from)`)  
 - `update_custom_active_authority`: Base fee similar to `account_update` plus dynamic one for any duration changes, no payback if duration in decreased.
 - `delete_custom_active_authority`: Cheap similar to `limit_order_cancel`