Looking For Simple Purge Of Records From A Table

greets all, ive got a table with batches of records. each group of records has a batch id as part of the PK in the form BTCXXXX where XXXX is an auto-incremented number. so lets say i have 100 batches of 20k records per batch in the table. so the distinct batch ids are BTC0200 (oldest batch) through BTC0300 (newest batch).
i only want to keep the 90 most recent batches in the table at any given time.
is it ok to just subtract 90 from the last batch id and do something like:

DECLARE @batch_id char(10)
SET @batch_id = 'BTC' + batch_num-90

DELETE FROM ITEM_BATCH
WHERE BATCH_ID < @batch_id

i want to cover if the table has more than 90 batches and if the table has less then 90 batches. is this a feasible approach?