Advanced timing at altitude

I've always used the old-school method of "earhole MK1" to set the timing on my vehicles. However, after recently replacing a camshaft, timing chain etc on my RV8 I put a recently-inherited timing light to the test, and found that the engine is best at about 18-20 deg BTDC! Although I live at 5000ft above sea level and use LRP (octane rating unknown), this seems very excessive. The engine will run if I retard it down to about 6deg BTDC but is very uncomfortable and suffers severely from lack of power. I'd be interested to hear whether such advanced timing is uncommon at this altitude. BTW I've checked the timing marks/pointer and they seem to be correctly placed.

How old is the timing light? What condition is it in?

I've had the misfortune to use more than one that were so inaccurate that the engine would hardly run!

Is it possible to compare it to one of known quality?

Funny you should raise that point - my wife asked the same question this morning (like she would have a clue!) Although the light is not new, its a very good quality one (not the usual incandescent type) and I've used it on my series Landy to get a comparison. Seems to be fine. The V8's has a good many miles under the belt, and I'm wondering whether the very advanced timing isn't perhaps just a combination of altitude, low octane fuel, and low(ish) compression. I read a posting on one of the American sites recently by a bloke who reckons that high up in the Rockies they run the big engines with extremely advances timing. I thought he'd made a mistake with the figures which he quoted, but maybe he's right. I just find it strange that my other vehicles don't behave the same way up here. Perhaps its time for me to consider doing up the rings and valves - should have done it when I has the cam out, but didn't have the time or inclination - oh well.......