My standard load with BP is between 70 - 90 grains of FFg.
With Obatex FFFg I would recommend a load of between 50 -60 grains of BP. You should rather use the 1.5Fg Obatex for the Bess. I would then use about 70 - 80 grains.

To give a bit of perspective, a load of 90 grains WANO FFG will give a velocity of about 1088 ft/sec using a .715" round ball (550 grains)