If you have measure metrics you can actually do this pretty easily.
First create several measurements and name them whatever you want (I usually put a number in front to organize them)
https://i.redgifs.com/i/lostrightxenurusunicinctus.jpg
https://i.redgifs.com/i/youngplasticamericanrobin.jpg
Tailor Tape 1 “01 Cock Anchor” – This will be the base for measuring the penis. Place the tailer tape at the base of the penis almost touching the stomach. You don’t have to set this to measure anything.
Tailor Tape 2 “02 Cock Girth” – Pretty self explanatory. This I usually put over the “shaft 4” node of futalicious (or dictator) genitalia. Set it to measure the “Futalicious Genitalia” node.
Next I make two tape measures for the length of the penis so that the measurement will be accurate even for bent, curved, or flaccid penises.
Tape Measure 1 “01 Cock Length 1” The first one I just copy and paste the transformation (not the rotation) of the cock girth tailor tape to put it in the middle of the cock over the shaft 4 node, but you can also just move it manually. Set it to measure the Cock Anchor tailor Tape. This gives us the first half of the penis length.
Tape Measure 2 “04 Cock Length 2” The second I copy the first’s position and them just move it out to the end of the glans until it’s just touching it. Set it to measure Cock length 1, giving us our second half of the cock length.
Compound Measure – “05 Cock Total Length” Finally select both cock lengths and create a compound measure. This will add the two measurements for a total measurement.
Finally it’s time to start parenting the different measurements to ensure they follow the penis pose. I do this last because if you parent them as you create them, it makes it difficult to locate them when you go to parent them.
01 Cock Anchor gets parented to either the Root or Shaft 1 nodes of futalicious genitalia. Whichever the tailor tape sits over more.
02 Cock Girth gets parented to Shaft 4
03 cock length 1 gets parented to shaft 4 as well
04 cock length 2 gets parented to shaft 7
The Total length measurement can just be parented to futalicious genitalia
You can even save this as a wearable preset (select only futalicious/dictator genitalia) and apply it to your characters and you won't have to go through this whole process again.
I hope this helps.