What if I want to order in increments – eg: if the lead time is 90 days but I want to review and order every 30 days

In this case you should set the ‘forecast period’ to 30 days and leave the lead time to 90 days. Just note that if you follow the suggestions it may take 90 days for the stock level to equalize to the optimal level, but after that you’ll get order suggestions for the next 30 days, but the buffer stock calculation will be based on the 90 day lead time.