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.
Tough question with limited information. Maybe you're leaning in, but your chair is 150 feet from your desk. In that case, you're probably too far away from your monitor. But if your chair is in a normal position, like, right next to your desk, then you're probably too close. The typical advice is to sit at least an arm's length away from your monitor.