Mojave bootcamp assistant limits partition size
If the BCA will not create the partitions in a way you want and you can, then consider installing W10 using a flash drive. While the use of a flash drive is not required, the procedure is simplified when one is employed. A Microsoft recommended install of W10 involves creating 3 new partitions which are listed below.


A Microsoft Reserved (MSR) partition of about 16 MB.
A Windows partition of at least 20 GB.
A recovery tools partition (WRE) of about 800 MB.


Even though Microsoft recommends a MSR, often the Boot Camp Assistant does not create this partition and Windows apparently works correctly when omitted. If the WRE partition is not created, then the software normally installed in this partition will be installed in the Windows partition. So technically, you only need the Windows partition. You can manually create the other partitions, but again this complicates the installation procedure. The exact Microsoft document explaining these requirements can be found at the Microsoft website UEFI/GPT-based hard drive partitions. Note: This document states the WRE should be 500 MB, but with the current W10 release, the size needs to be 800 MB. So, if you wish to create the three partitions, then the procedure would be as follows.


Boot from the flash drive.
When the first Windows 10 installation screen appears, enter a shift+F10 to open a Command Prompt window.
Run the diskpart command to create the partitions. In other words, create free space by deleting the single partition you created using macOS and create the three partitions in the free space.
Close the Command Prompt windows an proceed with the installation of Windows.


Instructions for creating a USB Windows 10 installer flash drive can be found as part of the accepted answer to Boot Camp Assistant fails to create the USB installer from the Windows 10 October 2018 Update.


  Note: When installing using a flash drive, the Mac may reboot back to macOS before Windows finishes installing. If this happens, the select Windows from the "Startup Disk" pane of the "System Preferences" application, then click on the "Restart" button. The Windows installation process will pick up from where it left off.


Steps to Download the Windows Support Software

This example as done using a iMac (21.5-inch, Late 2013) running macOS Mojave Version 10.14.4.


Using the Finder application, open the Applications folder. Next, open the Utilities folder, which can be found in the Application folder.
Open the Boot Camp Assistant, which can be found in the Utilities folder.
From the menu bar, select Action, then Download Windows Support Software.


In the popup window, select where to store the download. In the image shown below, the Downloads folder was selected.


Below is an image of what to expect during the download.


Below is an image of what is shown when downloading completes. Some of the Windows Support Software files may be needed during the installation of Windows. Therefore, even though you are suppose to run the Windows Support Software application (Setup.exe) after installing Windows, you still need to copy this software to the same flash drive containing the Windows ISO files.  




Installing Without the Windows Recovery Environment (WRE) Partition


  Note: You probably can skip step 2.



Boot from the Windows 10 flash drive.
In this case, you will not need to use the diskpart command. However, you still could use the diskpart command to view the partitioning of the drives. When the image below appears, press the key combination shift+F10 to open a Command Prompt Window. 



Enter the following commands.

diskpart
list disk
select disk 0
list partition
list volume
exit


Your results should be similar to what is shown in the image below.


  Note: The output from list disk command must show a * in the Gpt column for Disk 0.




After entering the exit command to close the Command Prompt window, proceed with the installation.
If you encounter the image shown below, select Custom: Install Windows only (advanced).


When the image shown below appears, select Drive 0 Partition 3. 


Confirm the selected partition is approximately 75 GB. Next, click on the Delete button, then click on the OK button in the popup window.


When the image shown below appears, select Drive 0 Unallocated Space.


Confirm the selected unallocated space is approximately 75 GB. Next, click on the New button, then click on the Apply button. Click on the OK button in the popup window.


The result should appear similar to what is shown below. Confirm the selected partition is approximately 75 GB. Click on the Next button to continue with the installation of Windows.


  Note: If you do not what a Microsoft Reserved (MSR) partition, then you can delete the partition before proceeding. In this example, I did not delete this partition.