[Boris]

In my humble opinion a lot depends upon brand with size calculators only giving appropriate starting size. 
Trial and error I have found to be the best way to bra shop if a fitting is not possible. Find a brand that suits and stay with that brand. 
In my experience lingerie sales people can be extremely helpful to the considerate shopper regardless of gender. 
I am sure many have been victims of the certain type of customer who gets pleasure in embarrassing staff.