Well, creating a gridsize of 81px is not a problem at all.
You can either set it initially when creating a new map, or afterwards when clicking on the stage and resetting the pixel value in the inspector on the right hand side of your screen.
I know what you mean regarding the zoom slider, it really could run a little smoother.
I would recommend testing things out before your session. By that I mean, what percentages you would need for your session, like;
you want to display the whole map - that's 4%,
you want to have the grid fit your 1" bases - that's 90%,
you want to show the players a certain part of your map - that's 45%, and so on.
You could also freeze the players view to have the zoom changes be more smooth for the players.
Setting up a checkbox for different media would be a little hard to do, since that's depending on your computers settings and the stats of the used monitor / TV. But I agree, a "scale to fit" setting for the players view would be an enhancement. I'll bring it up in our next meeting and drop a line on discord.
There are a few tutorials made by Board Games Simplified https://www.youtube.com/channel/UCbP3lV0E9fUzVfPmF46XMlw, but we know that we're currently lacking on proper manuals and tutorials, that's something we're about to change in the near future. While we're implementing DEIOS we wan't to build a wiki, or manual to have a database we can people point to when they need info about functionality.