You can't derive a boiler part load efficiency curve with just the nominal rated data for the boiler. Some boiler manufacturers do provide either graphs or tables that show the efficiency as a function of firing rate. Since the efficiency is above 82%, it is likely to be a condensing boiler. Unlike atmospheric boilers, condensing boilers are likely to be more efficient at part load. If more digging with that manufacturer won't provide more information, I would suggest you try to find something comparable from another manufacturer. With similar specs, boilers are not likely to be that different from each other.