For example every time I add posts I need to calculate by myself interval how much minutes I need to post like 10 posts a day(24hours)(1440mins/10posts=144mins).

I want just type how many posts I want to post a day every time I add a thread.
I know there is "time period in hours", but it is not like I want to. If it possible pls add.

"Time period in hours" does exactly that (it even uses the same formula internally as you wrote).

Select needed 10 posts -> use interval posting -> enter 24 (hours) in that field -> press "Adjust", the interval will be automatically set to 144 mins (the same as you just wrote).

If you selected 20 posts, just enter time period in hours as 48 hours (since you want 10 posts per 24 hours).

Edit: might actually add separate field for "posts per day" with "Adjust" button since it shouldn't take a lot of time.

why do I need 10 posts when I can add 300posts at once in thread. to waste my time?

You can select 300 posts and just enter 720 hours (which is 30 days - 10 posts per day) and it would calculate needed interval correctly after clicking "adjust".

2.50:
* Added new field in "Interval posting" window: "posts per day". Similarly to "time period in hours" it will adjust automatically based on interval or you can enter needed value and click "Adjust" button to automatically calculate the interval.