Best way to measure the working length of 21, with open apex is

When RCT is indicated in an immature tooth with wide open apex, a relatively reliable means of determining working length is by gently passing the blunt end of a paper point into the canal after profound anaesthesia has been achieved, the moisture or blood on the portion of the paper point that passes beyond the apex may be an estimation of working length or the junction between the root apex and the bone.