Nested if statements dependant on nulls
Methods should always be named in PascalCase.

You can condense the logic to the following:  

private decimal? SelectTolerance(decimal? globalTolerance, decimal? localTolerance)
{
    if (localTolerance == null || globalTolerance == null)
    {
        return localTolerance ?? globalTolerance;
    }
    return Math.Min(globalTolerance.Value, localTolerance.Value);
}