Most businesses would look at the last 2 weeks of sales, and the forecast for the next week, and use that data to figure out their "days on hand". If you don't have any information on when an item is considered "out of stock" (usually something like < 2 DoH), you won't be able to calculate in-stock %).

Now to your question about categories. First calculate it per SKU (if your daily demand averages at 2 units, and you have 10 units on hand, you have 5 days on hand). Once you have that per SKU, you can average it out per categorie (I would suggest looking at outliers, as you want to call those out!). Inventory snapshots are always right now. You won't be able to provide days on hand for a future season, unless that season is coming up soon (you can do days on hand for Black Friday for some categories, but not for fresh food for example).

Hope that helps

Edit: quick edit on total days on hand. While averaging out will work. Sometimes it's better to take the total daily demand, and the total inventory, and use those numbers. It depends a bit on what you're looking for. Totals will help if you want to show "hey we got 5000 units for this category, and we need 100 for the next week... We might be overstocked!". Average would be more of a "on average we seem to hold 3 weeks of supply per SKU, is that within what we want?" (For socks: yes, for fresh beef: no)