I am not an optician so can only speak from my own experience. I don't find any difference between 8.4 and 8.6 mm BC in a soft lens. The fact that there is very little choice available from the manufacturers (often only one BC and one Dia) suggests that apart from the economic considerations, one size is going to fit perhaps 90-95% of soft contact lens wearers.

There was a poster here a couple of weeks ago who had a very steep (short radius) BC and had some problems, but eventually went for a size bigger/flatter and I think found them OK.

The BC fitting/suitability is going to be assessed by the optician to ensure the lenses are able to float/move just enough and not too much. If they are too tight, they will cause problems and if too loose, may fall out/go off centre/constant refocussing as you blink? But that is probably only going to affect the few unfortunate people whose eyes are outside the norms in Base Curve, etc.

I don't think I had the radius of my eyes measured until a week ago when ordering RGP lenses.

RGP lenses are quite the opposite and must be fitted exactly - the BC fit can be tuned to correct a small amount of astigmatism, but of course RGP lenses are usually customised.

knotlob