Formulas Across Worksheets that are Dynamic and can Accomodate new sheets/conditions

Not sure if title makes sense but here is a problem I'd like to solve (as simply as possible). Lets just say I want to sum the same cell on a number of worksheets but I want to be able to add worksheets. That can be accomplished by putting sheets between two sheets called First:LastCell... at least that's what google told me and it worked. So if I add sheets between First and Last it will get captured in the formula. So what if I want to do the same thing but based on a condition in each sheet? As in I have a cell reference call out to either exclude or include for each sheet in the formula? Apologies if this is confusing.