My halyards are like Don's, going aft on the coachroof to the hatch, and also 3/8". I bought 77-78 feet for each main halyard (I have double sheaves) and have not cut them shorter.
For headsail sheets, make them at least twice the J measurement (foredeck) plus the distance to the winches plus enough for tailing. On the H25 this is about 40 ft.