Actually, you don't add these figures. The recommended PSU wattage to supply your system is just 250W. The online calculator suggests a 450W unit.

Your PSU is an overkill that leaves plenty room for upgrade. Staying within a very very safe margin, say 50% of PSU load of 350W (50%x700W) gives you a 100W (350W-250W) room. However, having a closer look at your PSU specs, I wouldn't recommend crossing a 60% load margin (leaving up to ~200W room for upgrade). Seems like your system has a capacity to support 2x medium-range graphic cards.

Moreover, as you have already noticed, your Intel i5-8600K processor has its own graphic processor. Your display can be setup to use it thus freeing your discrete GPU for rendering purposes only.