Rear wheel spacing/ cog position? measurement please

My current frame is actually an old steel MTB (yeah yeah I know, but it kinda evolved to SS, then 700c fixed gear wheels). Anyway you will be glad to know I am rectifying that and putting a road bike frame on it.

I had respaced my rear wheel to suit the wider frame and now need to respace it back to how it should be. (also I am currently gathering bits to build a new back wheel over christmas which will use a front disc hub with a bolt on cog respaced as a rear, yes I know its over kill and a properly fitted cog wont come off, but sometimes I like a bit of overkill, so I will want to space this wheel properly)

What should the distance as marked below be:

I say 18! Ami right?

The short question is "What should the OLD of my hub be?"

To which the short answer is "It should match the OLD of your frame."

Only you know that, but it will be either 126mm or 130mm if it's a road frame.

ah yes I guess the frame spacing is relevant (though not sure what OLD stands for?).

I measured the frame it measures 123mm, so I guess its a 120mm -with 1.5mm either side that would nip up when tightening. (it is a 1980 Peugout, it was 10 speed so 120mm would make sense). likewise if I build the wheel at 125 it would 'fit' .

I just want to get the cog lined up in a sensible place, then build & dish the wheel to suit.

Most fixed hub chainlines should be about 42mm ish to the centre, so that makes 18* left between the chainline and the over lock nut, given 120mm divided by 2 from your probable OLD. All this assumes you're wanting even undished wheel, as I guess you would with those hubs up there^. If you want to go 122/4mm to fit your frame better, make the hub flange to OL measurement more like 19 or 20mm.

Amiright?

*as someone said up there.

I liked the sound of the fixed MTB by the way.

(though not sure what OLD stands for?)

http://www.sheldonbrown.com/frame-spacing.html#spacing

Thanks everyone. I know what Im doing now.

I liked the sound of the fixed MTB by the way.

Its not really a fixed MTB though as it now has 700c wheels on it, it just still has an mtb frame. tires are 28c so on my none commuting rides I do a lot of gravel path stuff (farm tracks, converted railway lines etc, nothing hardcore I have another bike for that). info on the bike herehttp://www.lfgss.com/conversations/263535/#comment12139811 Ive just about got everything to do the frame swap now, need to add decals and lacquer first then time to build.