At 375 distributor rpm (750 engine rpm) you generally have about 0-2 degrees of mechanical advance coming in (assuming you are using a 1112403 distributor and a mechanical advance that is working properly). At 700 distributor rpm (1400 engine) you are looking at about 4-6 degrees mechanical advance. 1350 (2700 engine rpm) you should be at 9-11 degree of mechanical, and 1750 (3500 engine rpm) you are all in at 12-14 degrees. So at 900 engine rpm you are already 2 degrees advanced and maybe even a little more, so like sstock noted above setting the timing on the bb probably has you retarded a little.
But the mechanical advance is just part of the equation. The vacuum advance may or may not play a part here. The vacuum advance begins to start coming in when the diaphragm on the vacuum advance assembly sees about 4-6 inches of vacuum. You end up with a total of about 7.5 distributor degrees (15 engine degrees) when the vacuum reaches 7.5-10 inches of vacuum. The vacuum line is attached to the carb and the carb's vacuum port is actually located above the throttle plate, so its on the atmosphere side of the throttle blade as opposed to the engine side. When the blade moves and starts to open on acceleration, the vacuum port sees vacuum as air flows past the throttle blade and vacuum port. The vacuum level will increase as the throttle blade opens, then will decrease as the throttle gets closer to wide open. If you have the throttle blade cracked open far enough at idle the port may be seeing enough vacuum for the vacuum advance to come in a bit. So it could be that you have some mechanical advance and some vacuum advance coming in at your 900 degrees of idle which, again, probably has your timing a little retarded.
So, yes, as previously suggested, you should try to get your idle down to 5-6 hundred rpm. If you can, then the bb will get you at tdc. If not, use the above as a guide as to how much advance you may already have at 900 and make your timing adjustments accordingly. You might also consider removing and plugging the vacuum advance line when you are setting your initial timing. Once you have your initial timing set. you can connect the vacuum line, then start to add a little timing to see what your motor likes (if it pings you went to far). Every motor is a little different in what it likes timing wise, and gas and elevation above or below sea level also play a part so tinker away!
