Need help! Radiator (thermostat) bleeding bolt size


I mistakenly lost my thermostat bleeding bolt while doing radiator flush. I've been searching for it in my engine bay for hours. I cut my losses and put in some bolt I had laying around in my garage (12mm head). Don't know if it's the right length, but it stops the coolant from spewing out.

Anyone kind enough to tell me the correct length for the screw please? Thanks a lot