Not sure what type of boa you have, but I'm in the market for a scale and have pretty much figured that I'm investing in the "My Weigh 7001DX Precision Digital Scale with Weighing Bowl" from RBI (www.reptilebasics.com) which can be found at: http://www.reptilebasics.com/My-Weig...l-p-16191.html
I have seen it used by many breeders and it seems to be a good priced high quality investment. Also saying that, I can't weigh my boa with it. It only goes up to 7000 grams/15lbs. My boa is 6ft and around 14lbs but imagine trying to put him into something small enough to fit on that thing. I don't know maybe it's me, but I don't see that happening. Anywho, get you a scale and go from there. Sounds like she's too young/small to breed her. You can't just go by length and what she's eating in order to know if she's ready to breed. Good luck