Boiler Efficieny which is Used Under its Capacity

If I use a boiler (18 Ton/h 11.5 barg 185 C / natural gas) under its capacity (if it produce 10 ton/h steam, how does its efficiency change?
Is there a ratio or curve for this?
If you know your fuel flow at 18 tons vs 10 you should be able to make your own efficiency curve by calculating energy out (steam flow)/energy in (fuel flow).

18 tons/hr = 12.2MMBtu/hr
10 tons/hr = 6.8MMBtu/hr

Find the MMBtu/hr of fuel use for both cases and divide the steam energy by it.