I’m looking at lowering springs for my NBFL, currently has the 15’’ alloys that it came out the factory with, and I want them to fill the arches nicely, but not sure if I can accomplish this with such small wheels? and if so what size I should go for? i.e. 30mm, 35mm, 40mm etc.
Hi Hilldrith47, may I ask if you use the car on the road at all?
I ask because the answer may be important to how low you go.
Practically, lowered cars can be a bind to drive on a regular basis.
You might be surprised just how many routes have speed bumps, heavily distorted/subsided road surfaces or even in some places (I drive in and around rural Lincolnshire back roads at times) grass growing up through the middle.
A steep driveway can be a challenge. Even kerbs have been known to cause consternation. 
All of the above are likely to cause you grief if you encounter them in a significantly lowered car.
Having said that, one quick way to estimate a lowering value can be to place your open hand vertically with fingers together pointing into your wheel arch. With your hand parallel to the ground slide your fingers under the arch and see how many you need to touch the top of your tyre.
Then measure that and subtract one finger.
On my stock NB the original ride height was three fingers pushed under the rim of the wheel arch to the top of the tyre. Subtracting two gave a lowering value of 30mm (approx) to leave just a single finger gap between arch and tyre.
Visually the car looks better for sure but the OEM ride is compromised and fwiw, I sweat every time I see speed bumps 
Hth
Cheers, Guy
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Okay nice one, thank you a lot for the advice, I’m new to all this.
Fit coilovers so you can adjust the ride height to suit your needs perfectly. They do cost more but at least it takes away the guesswork of fixed height springs and if you do get it wrong your stuck with the new springs.
Buy bigger wheels to fill the gap?
That’s something I’m thinking of in the long run, just on a bit of budget you see mate
Yeah looked at a few sets of coil overs, could be worth the extra 2-300 quid, but most of them come with a 30mm minimum height anyways
Looks about right mate, probably gonna go with this one
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Love the look of that, what’s that wheel size?
16" Rota RBs
But a new set will give you a spare, winter, set.
Man maths 
Nah, you need to keep same overall radius or speedo reads wrong and larger tyre diameter also means body it will sit higher which is opposite of what you want.
I rather though it would be self evident a lower profile tyre would be fitted to maintain the gearing / rolling radius.
If you read the OPs post, the desire was to fill the arches, not lower the car per se. Lowering the car will disturb the asthetic of the wheel arch and wheel / tyre edge will will look rubbish as that two curves won’t match - imo.
but if the bigger wheels have lower profile tyres to keep the speedo correct they won’t fill the arches any more…
You’ve got me there…
Maybe ask those who go for bigger wheels to fill the acres?
I guess it could be the visual trick of a larger light disc (the wheel) filling the space, reducing the dark space (of the gap and tyre sidewall) around the wheel?