What is your boat? And what is your rig? (swept single spreaders? fractional?)
1.5 x (for/aft cross section of mast) or somewhere between 1%-2% is reasonable if you have swept spreader fractional rig.
It also depends on the cut and conditions of your mainsail. A few of the sailmaker have tools where you can take a careful picture of your sail from right below the boom (while sailing a a moderate breeze) and you can then calculate the depth/draft/twist at various points up your sail.
That can be helpful in determining whether you need more or less mast bend within the acceptable range. For example, and old baggy sail might benefit from more bend to help flatten out excessive draft. A new, very flat cut sail, might benefit from less pre-bend to add draft.
On my Legend 35, with a fractional/swept spreader rig, I easily have >= 8" of pre-bend (probably more). My backstay is adjustable and so I can further bend the upper 1/3 of the mast in strong conditions to further flatten and twist the top of my main and to dump a little from the top when overpowered.
Cheers
Chris
