I think it really depends how tight your bends are..
I would try to use a couple of coaches and two pieces of track to desired radius of tightest curve, and see how far apart the track needs to be before the coach end on one and the middle of the coach on the other 'DOESNT' touch.
That should give you a rough figure, and maybe add 10mm on to that for safety?
I'm in the same boat as you with the Hollywater line. 75mm seems to be the minimum I can do with 10 -12ft radius curves. I am looking at 80mm at present on the straight sections which is partly to do with the cross overs I have laid. I may then ease the distance to 85mm on the curves if I need to. I will be able to let you know in a few months once I am able to start laying the 'down' line.
I recommend looking at ' a garden railway odessy ' thread in the Layout progress which I found very helpful.