I'd hoped to borrow a mate's Blue Point/Snap-On one (0.1% accurate) but some bu66er's gone and nicked it from his tool store

I need to set the CO to 2.3 - 2.4% to run correctly and not wreck the valves/seats...
Anyone? I'm in Woking, Surrey but will travel a reasonable distance.
PS a Gunson one won't do, they're just not accurate enough, but I would use one at a pinch.