In my opinion. telling professor OK to ignore does not mean that he or she will. It only means Politely ask for the letter of recommendation at any convenience time. Besides, if you think it is obligation of a professor to write a LOR for student, then he/she will write whatever they think about you. My letter is short, yes, because not many of my professors are free of job. They have million things to do too. Please understand, the principle of asking for help is a
appreciation. This way, you have to show that you are in deep appreciation with your professor. And trust me, he or she will sit down and take thier times to write you a strong one. This is also one of the principles of Dale Carnagies in the Book "How to win friend and influence people". I recommend you read it.