I’ve already disassembled Shimano SG-X 50-34 50-F outer front chainring. Just curious, how could I pick up a compatible outer front chain ring for the crank Shimano 172.5 FC-4650 or there are not too many options for this type of the crankset?
The BCD is the diameter of the mounting hole circle - the only case where you could measure this through the distance between two mounting holes is when there is an even number of bolts and you are measuring between opposite holes. On a 5-bolt ring, the distance between two mounting holes will not be equal to the BCD.
But if you insist on measuring BCD from the distance between 2 mounting holes… it’s
BCD = L / sin (pi/n)
where n is the number of holes, and L the distance between two adjacent holes.
Of course, as the responses above have shown, searching for the crank part number is probably faster. And many chainrings have the BCD printed somewhere on them. And Shimano road is either 130 or 110 BCD, and the difference is visually obvious.