bpt,
Not a tznius issue; a sechel issue.
How presumptuious of you to ask someone to park your car. What is he.. valet?
When I pictured the scene described by the OP, I thought of a car halfway in a tight spot and inches from other parked cars, not a car in the middle of the aisle with a woman saying, “park this for me, please!”
The more I think about the situation, I think the best way to have handled it would have been to say, “is it possible you can tell me if I’m getting too close to the car over there? This is the only spot I can find and I don’t want to hit anyone’s car!!”
I have done this for others when watching them attempt to park near me and obviously having trouble seeing how close they are to the other cars.