I just spoke to my older brother who work at a transmission shop and has a lot of experience with the 545RFE and he said it really doesn't matter if you do it in park or neutral. He said if you have the correct amount of fluid in the trans either method will still fall in the cross hatches on the dip stick.

I have always went by the dipstick and checked it in neutral and never had an issue and have almost 126,000 miles on my XK. I'd personally rather go with what the dip stick says than an owner manual that has been known to have misprinted info in it.