Don't worry, I'm c**p at maths too.
AS350 is easy 10% = 20mins
Bell 206B = 25 USgals/Hr
Bell 205 100Lbs = 10Mins
120KTS= 2 miles a minute
Bingo, it works for me and I can't add up to save my life!
Fuel is, as said the most important thing without it you 'ain't goin' no-where!
PS. Try going to the eastern bloc countries where they give you height in meters and wind speed in meters per second, absolutley mind boggling