A difficult question as it depends on many factors.  The emmissivity of a flare depends not only on the MW (the higher the MW, the higher the emmissivity) but also the material.  For example, paraffins have a lower emissivity than unsaturated HCs.  

Also, the type of flare is a factor.  Sonic type flares are inherently smokeless and thus have a low emissivity.  A conventional flare, especially at low flowing rates if assist gas or steam isn't being used could be very smoky (as the flaring rate increases, the high velocity gas exiting the tip entrains more and more air thus reducing the smoke and its emissivity value even without steam or assist gas though that's not to imply it won't be smoky).

Because of these factors, some companies limit the lower level of the emissivity to ensure that ground level radiation isn't excessive, especially during an upset (eg. steam for the flare tip may be in short supply).  I've seen minimum numbers range from 0.3 to 0.4 explictly stating these limits over-ride vendor's 'expected' performance numbers.

If it was an inherently smokeless design, I'd be willing to go with the vendor's numbers after checking with some users.  If it is a conventional tip, I'd be more concerned.