Hi
There isn't a "Centre Frame" option, in the Crop effect, but you can use the Position effect to place your cropped and zoomed video exactly where you want. So reclick the large green + and add a Position effect.
The dotted image rectangle that now comes up in the Clip Preview window can be moved either by dragging. and you can adjust the effect by eye. As this is a bit hit and miss it might be better to adjust the Horizontal (Vertical) sliders (or enter values using the up/down buttons) monitoring the number of chequered squares there are to each side (top and bottom) of the image after each adjustment. They are pretty easy to count and adjustments using the up/down buttons are quite small each time.
If all your clips have been cropped and zoomed in the same way you can save your Position effect result as an Effect Chain Template by clicking the blue cassette button along the top of the effects pane. It will then be available to use on other videos treated in the same way in the Templates section of the Effects library.
If you have multiple clips on the timeline all requiring the same combination of effects you can, at the outset select them all (Click 1st/SHIFT/Click last.) Use the Video Effects tab on the top toolbar. When you have obtained your centered position click the 7th button along the top of the effects window (Looks like a bit of film with a small green cross) This will Append the effect chain to all other selected clips.
Nat