Hi @Jasper_Kooij,
First, to clear up one thing, the forum post referenced above (Rules set Custom Fields with Text or Numbers - #24 by Marie ) is referring to rule actions, not rule triggers, so it doesn’t have any relevance to your question here.
While Asana supports setting a value on a number custom field as a rule action, it doesn’t currently support using a number custom field in a rule trigger.
As @anon91170507 notes, our Flowsana integration does support number custom fields in its rule triggers, so that’s a possible solution for you.