Bob Allison
Hi Phil.
Guy Williams recommends 15mm for EM in his book on 4mm scale locos. I have used that on an LMS 4F which has an 8ft+8ft 6ins fixed wheelbase (it doesn’t get much worse) and it goes round my 3ft radius curves without a hiccup. The built-in 0.2mm gauge-widening with Gibson wheels and standard EM gauge track no doubt helps.
With 0.5mm frame material you can use Eileen’s 14mm x 0.5mm strip as spacers – but be warned they don’t take it to exhibitions unless you have ordered it in advance.
Regards,
Bob