Ehle Diffuser Adapter 18.8mm for 500ml bongs

Diffusor adapters by Ehle convince with their craftmanship and exact fitting. This diffusor adapter fits exactly onto the 500 ml bongs with 18.85 mm ground joint.
All parts from Ehle are handmade, therefore the length of the stem may vary. Certainly we take care with new orders that the individual parts fit. However, if you already have a 500 ml Ehle, please specify the exact required length from top edge (insert) to avoid complications.
How can I measure the right length of the downpipe?