Sales Projection Extension

Hello everyone,
It's a beginner in the PHP world, and I'm trying to discover how FrontAccounting is working. I didn't find any Starting Guide for Developer which is making it a bit hard for me to make changes in FrontAccounting Core System.

I've been given the task to create an extension for Sales Projection where our Sales Officer will show his projection on sales in his area. The extension will have his own separate tables to save the projection but it can get sales from current tables to match the projection and show the difference.

I also would like to make the changes in the core system of FronAccounting Purchase tab, to prevent it from adding GL Items into Supplier Total, instead, we want it to save the transaction in its respective General Ledger in Home Currency because those GL Items (in other words, Further Expenses) are from purchasing company.

I've succeeded in preventing it adding GL Items total into Supplier Invoice Total, but AJAX event on the current invoice page is still adding the GL total in Invoice total...

I don't find any forum for this topic so posting it here.

Regards