If your rear section is from a dealer I suppose it follows the Lotus short section of the time, which would starts behind the rear arches : I would go by that, as it leaves a rather convex curve on visible parts I find easier to align to than compound curves. If applicable, you may also want to check if there was a visible previous repair and take that in account if you think you should/can avoid piling up repairs (I like to make repairs as difficult to see as possible, inside out).
Of course, you may want to check dimensions before cutting of you car , just in case there would be a discrepency resulting in , say, a boot gap larger at the rear than close to the windshield… I’d make sure the chassis is up and stable on level ground, use lots of rulers to check curves and alignements simultaneously etc… for this kind of repair I usually build a wooden craddle to hold everything together before glueing, typically on a pallet…