The chop depends on the finished shape you want, if you want the "lakes' style of layed back 'a' pillars you dont need to section the roof just modify the 'a' pillars to lay back to the origional roof.
If you want to keep the proportions the same only lower you will have to cut the roof into three parts width ways at the rear of the 'a' pillar and centraly front to rear so you have 3 parts, add wege shaped peices of styrene untill all pillars line up correctly, then fill and file to finish the shaping of the lid.
To help you find any books like 'How to chop tops' by Tex Smith, chop a kit lid the same as a real lid to get the best results.