Is a manual log file grow procedure working different that autogrowth?
VLF Calculations
The number of the VLFs is not different for a manual or automatic growth of the transaction log file.
SQL Server &lt; 2014
Up to version 2014 of SQL Server the algorithm for the number of VLFs is as follows when you create, grow or auto-grow the TLog file:

Less than 1 MB : Complicated ignore
Up to 64 MB : 4 new VLFs
64 MB to 1 GB : 8 new VLFs
More then 1 GB : 16 new VLFs

SQL Server 2014 and higher
As of SQL Server 2014 and higher the algorithm is as follows:

Growth of TLog less then 1/8 th size of current TLog size?

Yes: Create 1 new VLF
No: Use the above formula (for previous version)



Your calculation for the number of VLFs should be correct.
Is your database set to AUTOSHRINK? Are you executing any other maintenance tasks that could be resizing the TLOG size?
Auto-growth: yes or no?
If you manually grow your TLog file then indeed you could turn off the auto-growth setting. But be aware: What will happen if you are unavailable and the database has to grow? The database will grind to a halt with an error message.
Baseline Size
At some point in time your database will have a TLog size that will have adequate space for the general imports that occur and the everyday workload and the TLog/Diff/Full backups occurring.
Use this baseline size as a starting point when you create a new TLog from scratch. If the database's TLog is really huge then create a TLog with a base size of 8 GB and manually grow in 8 GB steps until you have reached the &quot;baseline&quot; size.
Then leave some additional space on disk to allow for one or two &quot;auto-growths&quot; of 8GB size.
This will leave you with the best settings according to the article Transaction Log VLFs – too many or too few? where they recommend VLFs to be 512 MB in size.
Disk Space
If you don't have infinite disk space, then ensure that the TLog file has a maximum size set, that will not fill you disk to the maximum.
Monitoring
The most important point is to have a monitoring tool in place that monitors the database's &quot;free space&quot;, because you never know if a rogue process is going to do things you hadn't anticipated.
Transaction Log Physical Architecture
There is a good article on TechNet that explains the physical architecture of the Transaction Log. This gives you some insight as to why the TLog rolls over or why it might have to grow.
References
Important change to VLF creation algorithm in SQL Server 2014
Initial VLF sequence numbers and default log file size
Transaction Log VLFs – too many or too few?
Transaction Log Physical Architecture