Great question. Lots of ways to solve this. For YTD average you may want to be a bit more creative in the solution so you have more control over the math. You can try to use the YTD built in functionality but I think you'll find that difficult to work with.
So here's one idea:
I went ahead and used a variety of summary options like average, closing balance, max so you can see how they work. This gives you much more control over the time periods and removes the need to worry about the "current period" functionality.
If you need current period, then I would suggest setting that up in a system module and referring to it in a formula vs using the built in current period functionality. But that's just my opinion.