If plasma fibrinogen is less than 0.5g/l (50mg/dl) does that mean less chance of developing a deadly blood clot in a lung or other vein?

Why do you ask?: In coagulation, thing are never "everything else being equal." This value is about a third of the lower limit of the common reference range. Start figuring why. Was it diluted after fluid replacement after hemorrhage? Serious liver disease? Low from a genetic defect? A mutated fibrinogen may work but not be measured. Protection from blood clots is unlikely as this is driven by other factors.