Caculating Draw weight is iffy at best...most knowledgeable shooters could guess by drawing bow near as close...
Best is a bow scale...
I have a weigh scale that does over 100 lbs and i have a weight to calibrate it every once in a while...
Electronic or digital bow scales are surprizingly accurate..most Pro shops can get them for you...
Easton has a great one for around the $150.00 marks [most accurate]
Allan products has one for under $20.00 as does Cabelas...
Neil