private void handleBackwardSelection() {
		// value is a length
		int length = numberInputField.getIntValue();
		if (length == 0) {
			setStatusText("length must be > 0");
			return;
		}
		clearStatusText();

		Address currentAddress = navigatable.getLocation().getAddress();
		AddressSet addressSet = new AddressSet(navigatable.getSelection());
		if (addressSet.isEmpty()) {
			addressSet.addRange(currentAddress, currentAddress);
		}

		AddressRangeIterator aiter = addressSet.getAddressRanges();
		AddressSet newSet = new AddressSet();
		while (aiter.hasNext()) {
			AddressRange range = aiter.next();

			Address fromAddress = createBackwardToAddress(range.getMaxAddress(), length - 1);
			if (fromAddress != null) {
				newSet.addRange(fromAddress, range.getMaxAddress());
			}
		}
		ProgramSelection selection = new ProgramSelection(newSet);
		NavigationUtils.setSelection(tool, navigatable, selection);
	}
Class to set up dialog box that will enable the user
to set the available options for block selection
Ghidra/Features/Base/src/main/java/ghidra/app/plugin/core/select/SelectBlockDialog.java