The ride hight should be 170mm front and rear and should be within 5mm either way. If it's not, you have a problem that needs looking into. If everything is correct with your car, the ride height will fall within this tolerance.
To measure the right height you need half a tank of fuel in the car at the time of testing. Then you need two people weighting around 75kg (about 11.5 stone) each or 150 kg in total sitting in the car on a flat even surface. You then measure from the ground up to the chassis. This can be done at the front on the box section and at the rear at the bottom of the cradle.
If you fall below 165mm or above 175mm front or rear, you need to check all the suspension componants. Springs and dampers would be worn beyond acceptable limits, or you could have damage to the suspension mountings.
Below is the official stuff from Lotus
Suspension geometry adjustments.
Under normal service conditions, the only period scheduled check necessary is of the front wheel alignment. A full geometry check is required only after front suspension repair, or if any excessive tyre wear is evident, or if steering difficulties are encountered. Before any measurement or adjustments are made it is essential first to set the vehicle to it's "mid laden" ride height, approximating to driver and passenger (2x 75kg) and fuel tanks half full.
The mid laden ride height for the majority of the Esprit variants is 170 mm + or - 5.0 mm front and rear at production release. There are some exceptions, so please check owners handbook for geometry data. Some owners have none standard springs, increased their vehicle weight with accessories and some reducing their vehicle weight, the fitting of different engines, transmissions, wheels/tyres will have a bearing on "mid laden" ride height.
The dimension should be recorded below the chassis front section L&R and below the chassis rear loop. Replaced suspension components prior to geometry check should be left relaxed, steered wheels straight ahead, then secured at "mid laden" weight to specified torque. This practice is recommended to prevent pre-stressing and accelerated wear of the rubber bushes resulting in deterioration of the handling.