Re 'Export As' Can I have custom images sizes: 0.625x instead of the preset to achieve 50x50 from...

Really need someones help here.

Using the 'Export As' function to create two specific images sizes.

Main image needs to be 800x800 pixels. Second image needs to be 50x50 pixels, but the pre set options only allow for example 0.1x or 0.25x etc. I need to use 0.0625 to achieve 50x50 pixels from the main image of 800x800 pixels.

Help anyone please

Thanks Angus