When filing quarterly sales tax in California, there is a period when we must file for 1.5 months (May 1 - June 15). QB's Help had no answer. Any suggestions?

It's a great thing that you have found more efficient solution when paying SBOE and understand that you want a streamline process, Nicholas. I'll provide helpful information to make this clearer for you.

When dealing with the sales tax setup, you'll need to click Tax setting instead of the Manage Sales Tax option. Then, choose from the available filing frequencies: Monthly, Quarterly, Half-yearly or Yearly. 

Although QuickBooks Online (QBO) doesn’t currently support 1.5-month filing periods, you can continue using your existing workaround. If in case you want to update sales tax, check this article for more details: Edit Sales Tax Rate.

Also, QuickBooks sales tax reports can still provide the information you need to calculate and file accurately. You can generate the Sales Tax Liability report that provides detailed breakdown of sales tax collected and owed for a specific period and customize them to get the data you need. I've added this for you: Customize Reports.

If you need further assistance or have additional questions about sales tax or anything with filing, we're here to help.