I make a number format that doesn’t exist in standard LO Calc number format.
I often use this format, and often used on most files.
Is there a way to store my “user defined number format” so every time I create a new file, that my number format appear on number format list?
Maybe you can try to set up format number in blank document, save it as template, set up it as default template so every time you run Calc your number format will be loaded. And you can try to save it in Template folder (home/your name/templates) so you can create new file within Files with right click.
@Ljiljan, can you please post your comment in an answer so it can be marked as correct? I believe a template containing the required cell style (with required user format) is the recommended method. Styles in templates is the general approach to obtaining required formatting all types of document.
ok, I did it… thanks.
You can try to set up format number in blank document, save it as template, set up it as default template so every time you run Calc your number format will be loaded. And you can try to save it in Template folder (home/your name/templates) so you can create new file within Files with Right click → New Document.
Thank you. That is the best alternate way.
I wish someone from LO programmer/coder read this post and will add a new feature for future version of LO: save user-defined-number-format.
I understand you completely. I am using Impress and Draw more often, and I had to change border line and area fill each time I start doing something… so I created templates 
but Writer does not allow to change default border lines and area fill this way 
Your suggestion for workaround to this problem applicable for a new blank file. But to adjust already created files, it require more manual work.
BTW, I’m use Calc almost everyday, about 5 to 6 hours a day, and rarely use Impress , That’s why most of my post is related to LO Calc.
Anyway, thank you for suggestion.
For existing file it is very easy if ALL cells should have your special format. In this case do
If the format should be only for certain areas of a sheet, than you have to do it area by area.
I end up modify the number format manually area-by-area.
Modifying style (F11), will replace current cell’s background color, font, border, etc. While I just need to change only the number format.