The problem with the "book" is those timing specs were subject to emssion regulation/standards and not performance. If you don't have to test your vehicle for emissions, set the timing in the 10-16 BTDC range and see if the idle quality improves. You'll need to check total timing as well. Likely the distributor will need altering to keep total timing in bounds.
Search hillbilly timing tape if you don't want to get a timing tape. Haven't run a tape in years, because you'll need one or a marked balancer to set timing.
