Determine bolt size , do not use 1/2-13UNC (unless your design determines this size). Use 12 bolts and SAE Grade 8 mater

[answerhappygod]

Determine bolt size , do not use 1/2-13UNC (unless your design determines this size). Use 12 bolts and SAE Grade 8 material
Do not use preload requirements, use these:
Determine Preload max based on a maximum total bolt stress of 0.8 of bolt yeild strength
Determine Preload min based on a 1.3 margin x service load
There should be a ratio of at least 25% from mean preload to Preload min and mean preload to Preload max
Parts (a) and (b) are not required. Your preload calculations have addressed those requirements