Calculating Quantities in Order form

This is a really simple question but it isn't working well for me.

I have an order form with quantities and amounts.

I have a field called Quantity.0 and a field called Amount.0

I want whatever quantity is in the "quantity" field to calculate into the amount field.

However after creating simplified and custom, it keeps increasing my amount field as I go through my spreadsheet.

If someone wants to decrease their quantity on the form, I would like it to recalculate the amount off the new quantity.

Then at the bottom of my form, I have a total field which I want to calculate all the Amounts. I used the sum+ and reordered the fields but that isn't adding correctly.

Any help is requested.