Puzzles - Missing letters puzzles

Start with the top segment of the top left square, and move directly downwards. Each group of 4 letters follows the alphabetic sequence, in steps of 5 letters at a time. Taking other straight lines vertically and horizontally through the diagram, letters follow the same pattern.

Starting with the top left letter, and moving clockwise around the others, letters advance through the alphabet in steps given by the numerical value of the central letter (6).

In each group of 3 boxes, calculate the difference in numerical values between the top two letters, and add 1 to give the value of the letter in the lower box.