Hi. I've read elsewhere that a mould should typically be about 3 times the thickness of the part to be laid up in it.

I've never followed that train of thought. It's probably accurate to an extent, but I've made 15mm thick parts before that certainly didn't require a 45mm thick mould, that's unreal. I also don't think it works at the other end of the scale, I've laid up 1mm parts that needed a mould thicker than 3mm. Infact I would never make a mould only 3mm thick, that's too thin. 

Thanks for that. Interesting stuff. 

1 layer of 225 and then 4-5 layers of 450 is plenty if you're using tooling resin. That will give you a mould thickness of 5-6mm, which will be a very solid mould for most things. If you're using regular laminating resin without any fillers in it then you would probably want to do 6-8 layers of 450, and you would need to do it in 2 or 3 stages to prevent warping. 

Ah so the resin makes a difference...I was thinking of using regular epoxy (mainly to avoid the stink of poly/vinyl ester) and 600g roving in two lots of three layers?

Yes tooling resin has fillers in it. You can add some calcium carbonate to your resin to achieve a similar 'bulking' effect. 

I'm making some body panels for a car I will be building in the not too distant future. Still working on the patterns at the moment. This is a picture of the one for the rear panel. 

Very nice pattern work. 

So when you say corners do you mean inside or outside?

Try to keep your projects out of the bed room, I'm sure you love it but cross contamination from parts to bed sheets is not good. Keep it in the workshop 😉