They will give you a rough approximation. To be right on, they would have to duplicate the particular throat leade angles and diameters you are trying to match in your particular rifle. The chances of an exact match are pretty remote.
When I really want to find the the correct length for a rifle, I use the Sinclair tool that uses your chamber and a fired round from your chamber to calculate the "into the lands distance." I will then load a dummy round to that length and burnish the bullet with 4/0 steel wool and then check the land pattern on the bullet after chambering. Quite often I need to add .005" to .010" to the Sinclair tools measurement to get a square land imprint on a bullet.