You can access a tool to help you with calculating your taxes manually from the following link
http://www.revenuquebec.ca/en/sepf/services/sgp_winras/default.aspx
We have identified a problem with the latest update for Sage 50 2018.1 and Sage 50 2017.3.
The issue is with the Quebec taxes. The amount of tax being deducted is lower than what should be deducted according to the new tax rules with Revenue Quebec. Our developers are working on a solution to have a fix pushed out for this as soon as possible.
CAUTION: Sage support can't assist with third-party products, hardware, report customizations, or state and federal tax questions. Refer to our Scope of Support for more info. Contact your Sage business partner, network administrator, or accountant for assistance.
CAUTION: If you use third-party products that integrate with your Sage product, check with the vendor of your third-party product to ensure their product is fully compatible with this release. If you integrate with other Sage products, check with your business partner or Sage to ensure that these components are compatible. As a workaround we suggest the following