Did you find a solution? You can do this by taking the current PK and check number from the current form, searching the table for the current PK - 1, and taking the check number + 1 from that record and putting it in your CHEQUE_NUMBER field.
This means your PK and check number fields must be number fields (auto number for PK).
Do you need help doing this? If so, post the table information noted in the description above.