Here is a tutorial on pwm waveform generation for servos in software with other interrupts enabled and using only one timer overflow interrupt for duty cycle precision.
I currently have it running with +-16us precision. Greatest challenges has been getting it to run exactly with other interrupts getting in the way. This solution solved many of these issues.
Code is partly in the post, and partly in my avr micro firmware framework. Which, well, is currently at two places itself, one here: https://github.com/mkschreder/av... and one here: https://github.com/mkschreder/fo..... I will put them all in one place. That's zee plan. :=)