Originally Posted by
HillRider
One more time. The manufacturer's recommendations are based on your staying within their published limits for all parameters like max and min cog sizes, chainring sizes and total teeth. If you go outside their limits, you better be willing to adjust as needed.
Well the OP didn't say they were going outside spec. But your suggestion of big big +1 does happen to be what Shimano typically recommends for a 2x road bike setup. At least if that is without threading through the pully wheels.
Though most people going out of spec will be going for larger rear cog and I'd actually worry more about the slack when in the smalls in that case. But whether any DR can do less than big big + 1 I've never tried. And if going out of spec, I'd not go strictly by any mfr suggestion or rule of thumb. I just look and see what the results are. Probably by starting with the chain a link or so longer than the mfr suggestion.