I did an unscientific test years ago in an 87 SDL that showed no discernable difference going from D to P then checking vs first leaving the shifter in each position for a 3-count then back to P before checking.

Out of curiosity, what's the fluid level when cold? The FSM has specs for each model but 10-12mm below the min mark is common.

A personal thing - I aim for the middle of the min-max band when topping(!) any fluid in any car so there's a little room for error. What are the chances of finding an absolutely level spot on a round planet?

Sixto
87 300D