Why are you adding fluid? It shouldn't be low unless you have a leak.
You should expect to pay about $7 a quart, and you should add it through
the dipstick hole.
It doesn't matter which gear the car is in when it's added, but it should
be checked hot, idling in neutral. Not hot will result in a low reading,
not running will result in a high reading, and in park will result in a
high reading.