Baselinge grid alignment.

It's as simple as that your overall column height has to be an exact multiple of your leading value. (In any measurement system — but this is why many pros use picas/points, so that all measurements stemming from the type are inherently in step.)

The one variation is if you set a fixed first baseline of a different value than your leading, such as 18pt when your line spacing is 13pt. Then, of course, the column height has to accommodate this different value in the total.

Short version: leading, baselines and column height do not automatically line up. You have to do it in your page setup.

It’s hard to do that using mixed measurement systems (metric and points). Stick with using the same measurement for everything. that way you’ll know the live area for your text. Make sure the offset plus some multiple of the spacing is the same size as the text area.

What is the best way to have it aligned (or does it even matter)?

According to me, it is not very important. But if you actually want to  match the margin with the last line of the grid, measure the difference between the last line and the margin (x on my example) and add this value to the bottom margin.