This is one of those factors that need fine tuning to get the best from your load. I would suggest that you go by the OAL given in whatever reloading data you are using, and when you have got your most accurate round with that have a fiddle with the seating depth to see if you can improve matters. That is always assuming you think you need to improve matters.
Further to John's advice, the recommendation I've always worked to is to work out the maximum OAL for your rifle's chamber and then back off at least .010". If you reload to the maximum OAL and have to unload there is a danger - theoretical if not real - that the bullet will be gripped by the lands and unseat, dumping a whole load of powder into the action at best. By backing the OAL off at least .010" you should eliminate this danger.
To determine maximum OAL for my Sako 75 I used one of the RCBS Precision Mic tools:
Having got the starting measurement (max OAL - .010") you can then play around with other seating depths to see which give you the most accurate results.
However.....what you also need to bear in mind is the maximum length that will fit in your rifle's magazine. With my Sako, the most accurate OAL was too long for the magazine What I should have done in retrospect was to measure the maximum OAL for the magazine and worked down from there.
I'll touch on Willie's advice: If you are using the rounds for hunting in a repeater it is irrelevant where the lands are. You should start .005" shorter than magazine length and work towards SAAMI recommended OAL length for your cartridge. As John said, you need to zero in on the OAL. ~Muir
Oh, John?? Twice the distance from the middle to the end.
i made a dummy round no powder or primer,just so the bullet was held in the case then cambered the round and removered slow,i could see the rifling marks on the bullet. how much should i take off the measurement of that round to give me the best bullet seat beolw the OAL. or is it a case of trying a few and testing.