Thanks. Done some more investigation (using this thread). This works for me:
var int pulse
var numericC = (aqs_temp.state as QuantityType<Number>).toBigDecimal
pulse = (100 + (45 - numericC) * 180).intValue
Edit: var numericC = (aqs_temp.state as Number).DoubleValue
as suggested by @Udo_Hartmann fails with error The method or field DoubleValue is undefined for the type Number