The first screen snapshot below shows that I am in the process of creating a new ship design.  It shows that I have filled 78 of the 83 available slots leaving me with 5 yet to be filled.  The screen snapshot also shows that a Pulse Cannon has a mass of 5 meaning that I should be able add a Pulse Cannon to complete the design.  However, when I try to add a Pulse Cannon I get the message shown in the second screen snapshot below.

I understand the difference between integers and decimals.  If you are going to show integers then you should be using integer math.  If you are going to use decimals then you should be displaying decimals at the precision that matches component costs.