Giles,
That is only true if the centre of the radius is on the same centre as the RT. There are times when we have to go back to basics and start from the beginning.
Eric,
A lot depends if you have a chuck fitted to your RT or not, or whether the RT has a spigot hole in the centre (most do).
For a basic setup of the RT to spindle, you fit a spigot into the centre of the RT (you can buy blank MT spigots that usually fit, or if it is a straight bore, turn one up yourself). Then roughly centre it under the spindle using the table adjustments. Once there, mount a DTI in the spindle chuck or collet, bring it down so that the DTI can be pushed into contact with the protruding spigot. Then turn the spindle gently by hand so that it rotates the DTI around the spigot and mentally note the high and low spots as you go around, and adjust the table in small increments until you get very little deflection as you move the DTI around the spigot. Depending on how accurately you set the centre of the table inititially will depend on how many times you have to bring the DTI back into contact with the spigot. It does sound difficult, but once you get the idea, you can usually recentre your RT in about 5 mins.
Many people fit a 4 jaw independent on their RT. To me that is tantamount to self flagellation. If you can get a decent 3 or 4 jaw self centreing on there, and centred onto the RT using the centre spigot, that will be perfectly sufficient for most peoples needs. Use the same method of centreing as using a spigot, but come off a piece of round stock in the chuck. If you then want to be super accurate, you can fit soft jaws into the chuck and machine them in situ to fit the job size required (as long as it will fit on the jaws).
Maybe someone can try to explain it a bit better, but I hope this is enough to be going on with.
John