Ignition timing??

[bennwedge]

Hi all, I'm in the middle of rebuilding my 2lt cu a/c engine and have got to set the ignition timing , i've got a stroboscopic timing light but have no idea how to do it , I've got to set it to 7.5deg btdc at 800 to 950rev/min but what does the b from btdc stand for and where does the notch on the crankshaft pulley line up to on the timing scale if it was in tdc on cylinder1. Any help would be great cheers ben

[campervanman]

BTDC = Before Top Dead Centre