Your church may or may not be a great example, I'm not sure.
In part, the issue revolves around if people know what their tithes are going for and why they tithe. E.g. if they know the money is going to help starving kids (or whatever charity) and they give for that reason, then it makes sense to count tithes (or a percent of tithes?). If they would give tithes no matter what because they are supposed to, then it probably doesn't make sense to count them (at least if you are trying to analyze who is the most giving based on money spent or percentage of income, whatever).
In part, the issue revolves around if people know what their tithes are going for and why they tithe. E.g. if they know the money is going to help starving kids (or whatever charity) and they give for that reason, then it makes sense to count tithes (or a percent of tithes?). If they would give tithes no matter what because they are supposed to, then it probably doesn't make sense to count them (at least if you are trying to analyze who is the most giving based on money spent or percentage of income, whatever).