this works ...
func make_pretty_numbers( num:float, precission:int=2 ) -> String:
var unit:String
if num > 1000000000:
num /= 1000000000
unit = "giga"
elif num > 1000000:
num /= 1000000
unit = "mega"
elif num > 1000:
num /= 1000
unit = "kilo"
else:
unit = "base"
return str("%."+str(precission)+"f "+unit) % [num]