Returns the next lower or higher value in sourceField when there isn't a matching related value.
sourceField - the field from which the lookup value is taken
lower/higherFlag - the keywords lower or higher denote whether the value from the next lower/higher matching record must be taken if no related record is found
Data type returned 
text, number, date, time, timestamp, container
Originated in 
FileMaker Pro 7.0
Returns the value specified in sourceField using the relationships in the relationships graph. LookupNext is similar to Lookup, except that when the lookup fails, the value from sourceField in the lower or higher matching record will be returned, as specified by lower/higherFlag.
For this function to access the value in sourceField, the tables containing the source field and calculation field need to be related. Calculations using the LookupNext function won’t be forced to be unstored calculations.
Note  LookupNext returns ? when the related table is an ODBC data source.
In this example, you are shipping several items and the cost of shipping is based on weight ranges. Use the LookupNext function to find which shipping rate applies for an item. Use LookupNext with the higher flag instead of Lookup because the weight of an item may not exactly match the maximum weight, therefore we want to find the next highest value.
There are two tables, Items and Shipping Costs, in a database file containing data as shown below.
Items table
Shipping Costs table
The two tables are related by Weight and Max Weight. The calculation field Rate Lookup is defined as Rate Lookup = LookupNext(Shipping Costs::Max Weight; Higher).
The Rate Lookup calculation field will return 25, 25, 100, and 150 for records 1 to 4. Rate Lookup can then be used to get the correct rate code (A, A, C, and D respectively).
Related topics 
Functions reference (category list)
Functions reference (alphabetical list)
About formulas
About functions
Defining calculation fields
Using operators in formulas