A long line on a homemade score might mean a long sound, a loud sound or a movement across the room. If the performer must guess which meaning you intended, the drawing is asking one sign to do too many jobs.
Create a tiny score with two independent conventions. For example, horizontal length can indicate duration while a small written number indicates one of three comfortable loudness levels. Define the levels relative to the activity, never as an instruction to make an uncomfortably loud sound.
Draw three events. Make the middle event longer but quieter than the first. This combination checks whether your two signs can communicate different changes at the same time.
Ask for a reading before a performance
Give the score and its legend to a willing partner. Ask them to explain what they think should happen before making any sound. If their explanation differs from yours, revise the notation or accept their interpretation as a new version; do not quietly change the rules during performance.
Then perform the score with a gentle tap, hum or other suitable sound. Exact timing is unnecessary unless you have provided a counting system. Label the drawing as approximate if that is the intention.
Keep the original and revised legends together. The useful outcome is a score whose choices can be discussed: what is fixed, what is relative and what the performer may decide. A simple drawing becomes more expressive when its symbols stop competing for the same meaning.
Editorial illustration from the site library.