Given the variances in engine condition, fuel used and how (and even where) you drive I don't think any one number is the best for all.
Starting with the manual setting one usually has to experiment on the road to find what is ideal - generally the maximum advance you can get away with without knocking.
I set the distributor to TDC with the vernier scale on the big mark, then set advance from there with the adjustment wheel. That way I can always see at a glance where the timing is.