Does it have to perform at all? Most Duchesses I have flow have crap SE performance, just like most training aeroplanes (Seminole for ex).
I'd use the SE ceiling, as ceiling is defined as rate of climb of less than 100 fpm. If the SE ceiling is low, (4000' ish) as it is for many light twins then I'd assume it wouldn't climb. Some are better than others, for example the Seneca II will got o 13,000' on one engine by virtue of the turbos.
Of course if you drop below Vmc and you don't have height to loose the ONLY thing you can do is reduce power or else you are dead. Part of the FAA ME rating is to perform a Vmc demo (at height
) where you fail and engine, then go full power on the other and pitch up to let the speed decay to below Vmc. The thing that surprised me was the speed at which you loose it when it goes.