The High School Version of this:
I dont personally think that an interest level of 20% on her part would be her thinking your a stalker, for there to be any % above 20, she must like you a bit,
so here is my theory, i work on the basis of -100 to 100 i.e. -100 to 0 is all the negative stuff and once it goes to even 1% theres a bit of interest, therefore... the interest level is in proportion to how likely she is to say yes, if you asked her out...you have to get above 50% interest level for her to think your "dateable" here is my scale and tell me what you think....
0-10% She is aware of you and thinks your slightly attractive, she might get on with you but there is no spark to it and you are more of an acquaintence or a friend
10-25%She thinks your somewhat attractive, possibly dateable, but she doesnt like you enough to make a move or put any real effort into it, if you ask her out she wont say yes at this point.
25-50% She likes the way you act and wouldnt rule you out but still needs something to push you over the edge. Often the friends category, a zone that will not be easy to get out of and if the interest level stays between 25-50% for too long, it will never go above that 50% barrier.
50-60% She is more than likely to go out with you if you ask her, but she still isnt totally convinced, she knows your attractive to an extent and thinks you could be boyfriend material but the job is by no means done
60-75% She is beginning to like you more, shows interest and generally you can see this in the way she acts. If the interest is in this zone when you first meet her, your going to succeed, cos if you play your cards right, it will only increase.
75-85% You are probably about to, or actually dating her, she shows high interest and the first signs of infatution
85-95% Infatution
95-100% "Love"