Mack,
The method I use is as Follows and tools are cheap.
1 Buy a 1Metre length of suitable diameter of Dowelling from a DIY shop.
2 Cut an 8 inch length off (notice the change from Metric to Imperial).
3 Take your rifle and after ensuring that there is nothing live up the spout, insert the bolt and pull the trigger( this will give you the 15 thou. or so clearance from the lands for your cartridge OAL) exposing the firing pin.
4 Insert the long length of dowelling from the muzzle and mark with your knife, at the muzzle, when it rests on the firing pin.
5 Remove the bolt and with the muzzle of the rifle on the floor, drop your bullet of choice into the breech and touching the lands. With your 8 inch length of dowelling, just hold the bullet in place.
6 The tricky bit. Bring your rifle to the horizontal position and insert your long length of dowelling from the Muzzle again until it touches the nose of the bullet and once again mark the dowelling with a knife.
Finally, measure the distance between the two knife cuts with a Vernier guage and this will give you the Cartridge Overall Length. Simple.
This only takes a couple of minutes to do but the Cartridge OAL differs with each profile or weight of bullet. So it is worth recording the OAL for each make or weight of bullet used.