Amount of item with tax included (gross)?



I am currently switching from MyClientbase to Invoiceplane. I imported everything right, there is just one issue:
In Myclientbase I had the option to enter the amount of an item not including (default, like in Invoiceplane) or including the tax (gross). So Invoiceplane adds the tax another time to the amount which already includes the tax. Is there a way to switch that like in Myclientbase? Otherwise I have to correct lots of amounts which cause different total amounts because of the rounding.



Some of us don’t know MyClientbase.

Can you give an example of a mock invoice that helps understand better?


Myclientbase is the predecessor of Fusioninvoice and was also Open Source.
In the invoice itself, you won’t see that really.
It is just to enter a gross price . E.g. I have a „product“ which costs 4.49 per month including tax = 53.88 per year. If I enter the price without tax (3.77 rounded), the price per year is 53.83 including tax.

I have already seen other threads regarding this:

Is this so hard to implement? Because I’m thinking about implementing it myself (though I’m not really a developer). Could you tell me which files I need to change for that?
I would create a new database field called “item_tax_option”, 0 is without tax (default) and 1 is price included tax. Then in the frontend a new checkbox “Tax included”. An if tax included I have several price calculations to change. Would you tell me the files to be changed?