Since items are separated in their own reports combined in one via SubReport items, there is no setting to synchronize the size of items placed in separate reports. Also separate reports are not aware where the physical page ends, thus stretching the items to the bottom of the physical page is not supported.
In your approach you are relying on custom calculations for the number of pages and the items' sizes based on the report's physical dimensions and sizes, which may work in some custom cases but will require manual adjustments on a trial-error basis. The provided code is insufficient to determine what causes the issue. It might be related to the used font and its settings (as space per character and space around it), the saved items' sizes and locations, and/or the content (data) rendered in the report.
Details how items are positioned in the report can be found in Design Considerations for Report Item Layout
. Also it is recommended to serialize in a TRDX file
the modified in code report definition to check the produced report template with the Standalone Report Designer
On a side note, an image in the background can mimic that the item is continuing to the end of the physical page. The image can be set as report's BackgroundImage, where items can have specified their own background color to cover the image where needed.
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