1. You’d generally want apply your upscaling multiplication before rounding. On the other hand, you might also want an even number, as an image with uneven number pixels will be off-center by 1 pixel. Tbh, it will not make any discernible difference if you just go with even numbers.
2. TG-16 352 mode is listed in the linked page, looks like PAR there is 6:7. The 512 modes are just double speed 256. For Saturn I don’t remember, but this was talked about in another thread I believe. EDIT: dug it up, it has same PAR as TG16 352 mode (6:7): https://videogameperfection.com/forums/topic/saturn-optimal-settings/#post-20857