The diameter of some (not all) aftermarket springs that come with “coilover” shocks is based on industry standard widths. My Puredrives came with Pigtail springs, that is, a wide width at the top to fit ith Mazda top mount hardware, but a narrow end for the Protech spring perch.
Industry standard springs are (inner diameter) 1.9", 2.25" and 2.5". Spring rating, or weight, can be calculated from free length (the uncompressed length), the outer diameter, the number of active coils and wire thickness.
Cheap lowering springs are basically shorter springs, though the the good ones will have thicker wire. Cheap ones don’t and don’t last long, before they sag. Good ones have a variable rate, like FM springs.
Front springs are stiffer than the fronts, unless you have certain Spax kits, which are the same all round. You have to think about rear spring bias. For the NA MX5, Mazda set it at a little under 60% of the rating of the fronts. In Japan, they like these gymkhana type events, and to have sporty handling, you want rear springs to be a bit stiffer than stock, to induce oversteering behaviour. So the S-Special, which came with Bilsteins, has stiffer rears (and a bit of Antiroll bar tuning, fairly mild). In general, Japanese after market setups aim for this. Less rear spring bias (softer rears) is considered to offer better stabiilty for circuit racing. Some even disconnect the rear anti-rollbar for a little less oversteer.
In short, its really really easy to screw up the handling of the MX5 with poorly chosen components. Fitting adjustable coilovers to get a lower look is probably a really bad reason, because you will find drawbacks. Make sure you know twhat you are fitting.
One extreme example of terrible advice was experienced by a MX5OC member. He was a retired police driver, and could drive very fast. He was happy with the ride height of his UK spec Mk1; he fitted Konis and had taken a coil off the stock springs So it was very slightly lowered, maybe 14" wing lip to wheel centre. But he wanted to control the roll, so he called a well known supplier, to ask for professional advice, or so he thought. He wasn’t just sold thicker ARBs, but sold tubular ARBs, which are very stiff. Fantastic flat handling on B-roads, but he had no warning from the factory sized tyres on 14" wheels, when the bead started to seperate from the rim. So the car ended up in a ditch, on a road he was familiar with.
I’ve been a fair few setups over the past 27 years. From a budget set of Racing Beat Street springs on factory Konis, Japanese Apexis (lowering spring on fixed perch adjustable Gab shocks), looney Racing Gears (they were an experiment), anf Protech Puredrives. The one time I went with the herd was Meister Zeta Pros, and I wasn’t impressed. I even had a car with M2-Inc uprated stock Showa shocks (interesting ride quality).
Good luck with the Meisters. Look out for rust on the springs, at the top, where they can fracture. And speaking of industry standards, the Chinese shocks don’t conform to any. At least according to Iain Gardiner, who rebuilt my rear shocks a few years back. He has spares for AVO, Gaz, Protech, Koni, Bilstein. He has a business rebuilding vintage shocks. With Japanese shocks, he has no problem using seals etc from European brands, because the Japanese kept to recognised standards. He tried rebuilding some Chinese shocks (90% of them come from a white label factory, Bor Chuann, in Taiwan). No can do, nothing fitted.
The Puredrives came with English made springs. Flakey paint after 17 years, but quality steel. I had Meister Zeta Pros on another car; the springs didn’t last all that long before cracking right through at the top.