LP mixing valve has low flow problems could this be a turndown problem

[Redeyerick]

I've got a 1976 LP mixing valve that supplies our alternate fuel for the plant. It's volume rating is 80,000,000 BTU no other name plate data. Presently the plant demand is around 18 dekatherms, I'm having to flare off approximately 15 dekatherms to get the valve to control specific gravity. I'm guessing the turn down on this vintage is around 40 to 1? How can I determine the actual turn down using field data?