Well whoever Clay is.....he is wrong! 3 under puts more strain on the lower limb as you are further from the center of the string. If we could somehow hold the bow dead middle and pull the string evenly either side of the exact middle then there would be no need for positive/negative tiller. eg. a crossbow
The further you go away from the middle of the strings length the further you have to increase that limbs strength relative to the other. glassbows are good for testing
The other option rather than trying to make it such and such a measurement (which won't always be correct anyway) is to make your tillering tree so that the bows handle is supported but free to 'rock'. Then use a rope and pulley for the pulling string. Draw a line straight down from the point the arrow touches the bow. Clip (small karabiner) your pulley rope to the string right where you will be pulling from.
Now as you tiller your bow the 'pulling point' krab is free to move left or right dependant on the limbs relative strength to each other. If you have the bow balanced perfectly the 'pulling point' on the string will follow the straight line you draw. If not then it will drift towards the stiff side. This balance as you draw is key to making a well manared bow that will last and last......because it was made balanced in the first place
I can take a photo of my set-up if you want. It is well worth the time to make a set-up like this as you can tiller each and every bow, of whatever design, perfectly. A lot of handshock ,thuds,twangs and buzzing can easily be avoided
Once I made my set-up it confirmed a whole lot f stuff regarding tillering for me.