Question

sterochemistry

Hello I have a question about sterochemistry. When trying to figure out if two fisher projections are the same i turn them into bond line structures. When i number the carbon chain according to IUPAC rules i sometimes get stuck on questions where regardless of the direction i number the chain it will be the same (for example if i number one structure from left to right and the other from right to left i'd get the same thing), now my question is when i assign R and S depending which carbon i use for my reference it can determine whether i get the question right or wrong. Do i number my chain in the same direction when comparing two structures? or do i just have to find a direction for each that would make them "same"?