The best website I know of for measuring accurately is https://www.abrathatfits.org/calculator.php it will ask for slightly more information like leaning forward to be more accurate reading.
For your second question some brands can have up to 3 levels https://www.shyaway.com/wardrobe/complete-push-up-bra-guide/ usually you want to follow this with the current size you are this can very still between brands, cheaper off the rack stuff might all be level 1 at best and not give desired lift. I'd suggest not going down a size and it might be too tight and hurt.
Have fun <3
For your second question some brands can have up to 3 levels https://www.shyaway.com/wardrobe/complete-push-up-bra-guide/ usually you want to follow this with the current size you are this can very still between brands, cheaper off the rack stuff might all be level 1 at best and not give desired lift. I'd suggest not going down a size and it might be too tight and hurt.
Have fun <3