Tap-drill discussions often use a percentage of thread as shorthand for radial engagement. Joint design also depends on engaged length, internal and external material strength, load direction and failure mode.
Why tap-drill size changes torque
A smaller pre-hole leaves more material for the tap to remove or displace. The resulting increase in thread height may add little useful joint strength while increasing torque, chip load and breakage risk.
Why engaged length matters
Load is not shared perfectly equally by every turn. Material stiffness, geometry and manufacturing variation influence the distribution. Use an applicable design method rather than multiplying a single-thread value by the turn count.
A practical process view
Select the pre-hole from tool-maker guidance for the required thread and material, control the produced hole, and inspect the finished feature. Treat joint-strength validation as a separate engineering task.
Do not derive load capacity or safety factors from a general tap-drill percentage rule.