Is it true when talking about values in
[Memory used by applications] = MemTotal - (MemFree + Buffers + Cached + SwapCached)
Yes, what you are saying is essentially correct. That is one rough way to get the memory used by applications.
The formula holds true, nonetheless.
A great article breaking down meminfo can be found on RedHat's site here
For the record:
MemTotal: Total usable ram (i.e. physical ram minus a few reserved bits and the kernel binary code).
Also, its not that exact of a science when figuring out available and used memory. Linux caches all the memory it can and uses it as needed.
Free -m will help display the current memory usage in MB
It would also be beneficial to visit LinuxAteMyRAM