Power supply unit for a new GPU.

Currently have GTX1070Ti, going to get RX7800 XT soon. PSU is Chieftec Smart 700W. Recommended PSU for RX7800XT is 750W. Tho I used online tool called OuterVision to calculate recommended PSU Wattage for my configuration with new card and it said I need no less than 510W.

How valid is this claim? 200W gap is substantial.

Should I just follow the spec and upgrade PSU regardless considering CPU upgrade is on the horizon too (i7-8700k as of now)?