If your designing the spring in detail then you can use the equations reference and as observed it is very sensitive to wire diameter.
However if you are just trying to select the free length of a 160 lb/in spring compared to the free length of a 110 Lb/in standard spring it is much easier.
On a Plus 2 according to the manual the standard front spring compresses from 14.19 free length to 8.6 inches at normal ride height. This is a load of 615 lbs. A 160 lb/in spring needs to compress 3.84 inches to achieve the same load. To achieve the same length at normal ride height the 160 lb/in spring needs to be 8.6 plus 3.84 inch to give a free length = 12.44 inches.
Some later cars in some markets used a longer free length of 14.59 inch to give a longer fitted length of 9 inch for a higher ride height.
If you want to lower the car a little then you can use a shorter free length to achieve a shorter fitted length and lower the car. I lower my Elan for competition about 5 to 10 mm on the suspension and the rest (about 30 mm) through lower profile tyres.
If the regulations don't allow adjustable spring perches just glue them up once you have set the ride height. I bet the regulations don't define what non adjustable means in detail
cheers
Rohan