Drop bar levers have an actuation ratio of about 6 : 1 (IIRC the actual number is 5.6), flat bar levers have about half that.
It's unusual to set up brakes with more than 30 mm of lever movement, so in practice you can count on about 5mm of cable pull for drop bar levers and about 10mm for flat bar. It's not hard to determine how much cable pull a given brake needs, if it's closer to 5mm than 10 your levers should be good to go.