You do not "calculate" any loop lengths. You make use of a timer. A timer will - besides running at a fixed and known speed - also take into account any time spent in interrupt handlers. A busy loop (C or assembler) will not know if any ISR has been active.

A timer can also allow longer delays to be handled while the processor spends time doing real work, such as processing received data on the serial port.

For short loops, you can do a busy-loop in assembler. But the busy loop will only guarantee a "minimum" delay, unless you run it with interrupts disabled.