I design for printing and use 'Save for web and devices' to have crisp images.
E.g. I can have document 300x900mm, and then I export it as 600x1800px and have my snapped sharp lines to stay sharp.
But 'SfWaD' always immediately renders image at '100%' (which is not 100% at all at Windows — it's an another problem to be discussed), and I get 850x2551.
When my dimensions are bigger, AI sometimes can't even render an image and I get error message and pink fill in preview area.
I try to draw ten times smaller whenever is possible to avoid this, but sometimes it's not convenient.

'Export for screens' doesn't provide direct custom scaling, only integers.

Proposed solution is too remember scaling in 'SfWaD'. In example given it could stick to the 70,6% after pressing 'Done' button.
But it will still require AI to render at 100% anyway, so the second better way is to skip immediate rendering and letting user to set values first (you can set an option for it).