Currently rules only affect custom fields that are Dropdowns. The ability to use rules to affect Numerical or Textual Custom Fields would be wonderful.
Welcome to the Forum @John_Lathbury!
Rules are very new and we’re planning to improve them in the future so thanks for taking the time to share your thoughts with us!
In the meantime, I recommend you to have a look at Flowsana developed by @Phil_Seeman. This integration supports both conditions/triggers and actions using all types of custom fields. You can find more information in the following post: Now rolling out to Asana users — Rules
Have a great weekend John!