have smth. at one's fingers' ends

(finger-ends, finger tips или finger-tips)

   знaть чтo-л. кaк cвoи пять пaльцeв, влaдeть чём-л. в coвepшeнcтвe; coбaку cъecть нa чём-л. Vicar. I see you have the usual Rationalistic arguments at your fingers' ends (W. S. Maugham). He had the scientific world at his finger-tips, and was one of the shrewdest men I knew (C. P. Snow)