I see that you've not had any responses to your request so will suggest that you consider a non-FMP solution.
It is MoneyWorks, a full accounting package that has an excellent (and free) plugin for moving data to and from FileMaker. We have used this package for over 15 years, both as our main bookkeeping program, and as a Debtors module for the MailOrder side of our business.
The developers of this package know FMP well and thus it is not surprising to see what looks just like FileMaker's calculation engine available inside of MoneyWorks.
If you look back in the Archives for the thread on MoneyWorks and FileMaker Integration (https://fmdev.filemaker.com/message/84295#84295 ) you can read some of Grant Cowie's recommendations on the integration process.
Several years ago Linda Pitts did a comparison of the effort required to integrate five different bookkeeping packages with a FileMaker solution. She found that MoneyWorks was by far the easiest and wrote this up as an article for FileMaker Advisor.
I hope this helps,
I'll check this out.