That's absolutely brilliant drystonepaul!
My first MTB was the DB Ascent EX you have there. The Holdsworth too, with the 42:16 is very encouraging, because I'm using a 43:17 set up.
I've kept the moving parts to 1/8 specific with an Izumi track chain, Halo Fat Foot and Velo Solo front ring.
I will definitely go with the Magic Ratio as my first port of call. I've ordered a ZTTO tensioner as a backup.
The build is heavy on the aesthetics and I will post pictures soon of the build. Its Campag heavy outside of the above mentioned parts.
Everyone's advice here is proving invaluable.
If you are familiar with measuring ratios in gear inches, then somewhere around 70 gear inches is just about perfect for road use as long as gradients aren't particularly steep.
42:16 gives you just under 70 inches with 700c wheels and 43:17 should put you around 68 inches.
My main singlespeed normally runs a 42:17 setup giving about 67 inches.
I've use that for rides up to 200km as the slightly lower gearing is useful and preferable on longer rides.
For off road it's better to have lower gearing.
On 26 inch wheels a 2:1 ratio or lower tends to work well. So 36:18, 34:17 or 32:16 all give the same 52 gear inches or thereabouts.
Tyre size makes a difference. Bigger tyres increase the wheel diameter and hence give a slightly higher gear.
650B and 29ers give higher gearing so require different combinations for the same given 'gear'.
Once you get bitten by the bug of single speeding, having a 16,17 and 18 tooth sprocket will allow for a bit of experimentation.
Using a tensioner will give you versatility on any frame with vertical dropouts.
Geography and terrain are also a factor. I live on the edge of the Peak District to the east with the Cheshire Plain to the west.
One gear wandering is great on the flat lands, but can be a real challenge once you start hitting the hills.
