personally I would continue to level up until you actually start completing unassisted pullups then begin with level one. Everytime you do the pullups however I would put a solid effort to do a pullup each time without cheating then complete the cheating pullup. Its kind of a pain in the ass but your strength and endurance gains will improve. You might try body rows, jumping pullups, negatives, etc. in addition to the 1 leg assisted pullup.