You can use ^ and %, but must enter them in the _Alphanumeric variable twice as ^^ or %%. However, if you want to use the result (_RndAlphaNum) later in the batch file (other than Echoing to the screen), they might require special handling. You can even use a space, as long as it's not the last character in the string.
23 inches (59cm) is the minimum distance required for a chair behind a desk. This would fit a modest sized chair. For more comfort and to accommodate a bigger office chair increase the chair clearance to 30 inches (76cm). Office Chair tips. Swivel chairs make it easier to get in and out from behind your desk, especially if the space is a bit tight.
The rule of thumb that can vary a bit but should be considered is the height of the seat should be 12 lower than the table. Most tables and desks are 29 -30 high. Placing chairs at about 16 -18 high. Bar height is 42 making most stools around 30 high. So your benches all seem to fit into that chair category of 16 -18 high. The depth would be a design choice rather than comfort if there is no back on it.