I went through this process recently and may be able to save someone some trouble.
The elongation method is a pain get the Loos gauge from the start.
This process requires patients. don't try to set the mast in column and lock it down at the dock. It wont work. The only way to get it right is to get the mast centered in the boat with the right rake and prebend and then do the rest undersail.
to do this right you can count on it taking forever. To get no wind and no wave action to get the mast in column at the dock is hard. then doing the real tuning under sail in the wind speed you want to tune for may require several outings and a lot of tacking.
Don't kid yourself that you can put the mast in collum and lock it down at the dock. the real tuning part can only be done under sail, with a load on the mast and stays. There is no short cut here!
I tuned my rig for twenty knots I figured that was good middle ground for us.
And did not exceed 15% on the lowers 20% on the caps.
I maybe the exception, Our boat was layed up for a long time when we bought it and I don't believe the rig was kept in tune pryer to that so i had a hard time getting things adjusted. I tried moving the mast with the stays at first and finally just loosened everything and started from scratch. And followed a procedure much like the the one Giu. posted.
Then after all the headache. and some help from Alex I finally got it right. I can drive with my finger tips on the wheel, my weather helm went down and the boat points higher and is faster.
A couple things that I found out were that ambient temperature has a big effect on tension and I have a hatch just aft of the mast. It was a lot easier to sight up the mast from the salon looking up through the hatch than trying to to do it at the base of the mast.
When the rig is right it makes sail trim and helm balance so much easier.
I look at it like this. if the foundation is crooked the house is crooked. There is no way you can get proper sail shape with the rig being out of wack.
If the center of the mast is falling off to leward then you cant flatten your main in heavier air. just when you want to depower the sail it's gettig powered up. The same as running loose back stays will add power to a jib.
It was an enjoyable process for me the results were well worth the effort.
Hope this helps
s\v Soul Searcher
All boats are sinking it's just a matter of how fast.