What is the minimum PA a batter must have to not go below 100% fatigue...600? Higher? Lower?

Thank you in advance.

Can't say with certainty. Here's one estimate:

Your leadoff hitter will get 720PA if he plays all 162 games. Each successive spot in the batting order will get ~18 fewer PA.

This is purely an estimate - someone (zubinsum perhaps?) has posted the actual average PA by batting order position before. But keep in mind these are averages. They will vary based on salary cap, home park, quality of your hitters (the better your offense, the more PA each player will get), etc.

But assuming the 720 is reasonably close, then if your leadoff hitter had ~660PA/162, he should be able to play a full season at 100% fatigue. And each spot in the batting order will need 15-20 fewer.

Having said all that, you'll notice most successful owners draft fewer than the # of necessary PA to maintain all starters at 100% fatigue. Why? Well, if you have AAA, then you have players who can give you ~200PA apiece, and maybe more. Plus, you have to spend some of your salary on a bench...if you have to pay for those PAs, you may want to use them. Finally, many owners are comfortable letting their players go below 100. There's no hard and fast rule for how low you can go, but some owners are comfortable with 97; others 95; others 90, etc.

39.6 PA/g * 162 g = 6415 PA per team per season.

Divide by 1.1 to account for the 10% bonus which gives 5832 PA per team per season.

Divide by 9 to give an average of 648 per lineup spot.

That gives you close to the correct amount for your #5 spot. Now add about 20-25 PA per spot up in the lineup. That gives 728-748 for a leadoff hitter.

Keep in mind, this is only an estimate. Actual needs will vary greatly according to line-up structure, ball-park and offensive salary cap. Generaly speaking, the better your offense, the more PA you need.