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Over the years, it has been a popular product suggestion request to have Life Insurance to allow 3 decimals, as premiums often go out to 3 decimal places, and the SQL Table does allow up to 5 decimals.

You can change the decimals under Tools | setup | Payroll | Payroll | Options for benefits/deductions to go to 3 decimals, however this changes ALL benefits and deductions in HR and Payroll, and users want it for Life insurance codes only.

(Note: Older product suggestion databases would have more votes on this, as it has been a popular request over the years. This post below was logged, but I think it was worded incorrectly as the ability to go to 5 decimals is already there and users want it for LIFE INSURANCE ONLY and not 'all' ben/ded codes: )
https://experience.dynamics.com/ideas/idea/?ideaid=55806895-a627-e811-bbd4-0003ff68bdb9
Category: Benefits
STATUS DETAILS
Declined
Ideas Administrator

This is not something we would consider as it would be a larger change for Dynamics GP for the needed customers that are requesting it.
Thanks
Terry Heley 
Microsoft