I use 3 methods, well really 2 cause I don't use the Sinclair anymore. One is the Hornady OAL gauge and the other is the R-P Products rod which works like the cleaning rod method described above. Here is a pic of the Hornady with the R-P rod
The R-P rod is a stainless rod with 2 lockable collets and a brass tip. You just insert the rod down the muzzle to the bolt face and lock the rear collet
then insert the bullet in the leade and hold it there either with the Hornady OAL Gauge or a wooden dowel
insert the rod down to the bullet tip and lock the front collet
then measure between the collets
Works everytime and you don't have to worry about how much headspace your modified case has.